The Five Ways: Aquinas on Whether God Exists (Summa, First Part, Question 2)

Lesson 02 told the story of Aquinas's life and lesson 03 showed how every article of the Summa is built: the question beginning "Whether," the objections, "On the contrary," "I answer that," and the replies. This lesson applies that method to the most read three pages of the whole work. Readers who have not yet met the five parts of an article should read lesson 03 first, because ST I, q. 2 follows the pattern exactly, and a good deal of what is said about the Five Ways in popular books comes from reading the body of article 3 without its objections, its replies, or the two articles before it.

All English quotations from the Summa in this lesson come from the translation "literally translated by Fathers of the English Dominican Province," second and revised edition, 1920, as given online at New Advent, which prints that edition's title page and approval at the foot of every question.¹ The printed translation uses a few long dashes; in keeping with the style of this site they are shown here as commas or colons, and no word has been changed. The Latin comes from the Corpus Thomisticum, which reproduces the Leonine text.² Each citation gives the part, question and article in the standard form (for example ST I, q. 2, a. 3, or ST I, q. 46, a. 2, ad 7 for the reply to the seventh objection) and links the page for that question.

Where question 2 sits in the Summa

Question 1 of the First Part asks what "sacred doctrine" is: whether it is needed besides philosophy, whether it is a science, whether it argues, and how Scripture is to be read. Lesson 03 worked through two of its articles. Question 2 opens the treatise on God, and Aquinas begins it with a short paragraph that lays out the plan of the whole Summa. The New Advent page for question 2 omits that paragraph; the same 1920 translation as given by the Christian Classics Ethereal Library prints it:

"Because the chief aim of sacred doctrine is to teach the knowledge of God, not only as He is in Himself, but also as He is the beginning of things and their last end, and especially of rational creatures, as is clear from what has been already said, therefore, in our endeavor to expound this science, we shall treat: (1) Of God; (2) Of the rational creature's advance towards God; (3) Of Christ, Who as man, is our way to God. In treating of God there will be a threefold division, for we shall consider: (1) Whatever concerns the Divine Essence; (2) Whatever concerns the distinctions of Persons; (3) Whatever concerns the procession of creatures from Him. Concerning the Divine Essence, we must consider: (1) Whether God exists? (2) The manner of His existence, or, rather, what is NOT the manner of His existence; (3) Whatever concerns His operations, namely, His knowledge, will, power. Concerning the first, there are three points of inquiry: (1) Whether the proposition 'God exists' is self-evident? (2) Whether it is demonstrable? (3) Whether God exists?"³

Three things follow from this paragraph, and each one matters for reading the Five Ways. First, the question about God's existence is placed inside "sacred doctrine," the teaching that rests on revelation, which question 1 has just described. The Five Ways are part of a theology textbook; they are not a separate tract written to win over unbelievers, though they have often been printed and taught that way. Second, the question "Whether God exists?" comes before any question about what God is, and the very next question (q. 3) will begin to say what God is not: not a body, not composed of matter and form, not a member of any genus. Whatever the word "God" means at the end of each of the Five Ways, it will be filled out over the next twenty-four questions. Third, the order "self-evident? demonstrable? does He exist?" is logical. If "God exists" were self-evident, no demonstration would be needed; if it could not be demonstrated, article 3 could not offer one. Articles 1 and 2 clear the ground for article 3.

As lesson 02 set out, the First Part was begun at Rome, at the Dominican house of Santa Sabina, in the years about 1265 to 1268, when Aquinas was teaching the young friars of his own province. The Summa contra Gentiles, finished a few years earlier, had already set out arguments for God's existence at much greater length in its first book, chapter 13. That chapter will be quoted below, because there Aquinas names the sources he drew on, which he does not do in the compressed article of the Summa Theologiae.

Article 1: Is "God exists" self-evident?

"Self-evident" (Latin per se notum, "known through itself") is a technical term. A self-evident proposition is one whose truth is seen as soon as its terms are understood, such as "every whole is greater than its part." If "God exists" were self-evident in that sense, then anyone who understood the words would know it was true, and no argument would be needed or even possible. The article sets out three reasons for thinking it is self-evident.

The three objections

"Objection 1. It seems that the existence of God is self-evident. Now those things are said to be self-evident to us the knowledge of which is naturally implanted in us, as we can see in regard to first principles. But as Damascene says (De Fide Orth. i, 1,3), 'the knowledge of God is naturally implanted in all.' Therefore the existence of God is self-evident.

"Objection 2. Further, those things are said to be self-evident which are known as soon as the terms are known, which the Philosopher (1 Poster. iii) says is true of the first principles of demonstration. Thus, when the nature of a whole and of a part is known, it is at once recognized that every whole is greater than its part. But as soon as the signification of the word 'God' is understood, it is at once seen that God exists. For by this word is signified that thing than which nothing greater can be conceived. But that which exists actually and mentally is greater than that which exists only mentally. Therefore, since as soon as the word 'God' is understood it exists mentally, it also follows that it exists actually. Therefore the proposition 'God exists' is self-evident.

"Objection 3. Further, the existence of truth is self-evident. For whoever denies the existence of truth grants that truth does not exist: and, if truth does not exist, then the proposition 'Truth does not exist' is true: and if there is anything true, there must be truth. But God is truth itself: 'I am the way, the truth, and the life' (John 14:6) Therefore 'God exists' is self-evident."¹

The first objection comes from John of Damascus (about 675 to 749), the Greek monk whose On the Orthodox Faith was read in Latin in the West; "Damascene" is the medieval Latin name for him, and Orthodox readers will recognize him as one of their own Fathers. The second objection is Anselm's argument, though Aquinas does not name Anselm. The third is an argument from truth, which Augustine had used in his own way. Aquinas thus begins by stating, in the strongest form he can, three reasons that came to him from Greek, Latin and philosophical authorities.

Anselm's argument in his own words

Anselm (1033 to 1109), abbot of Bec in Normandy and later archbishop of Canterbury, set out the argument in the second chapter of his Proslogion, a book written as an address to God. In Sidney Norton Deane's translation (Chicago, Open Court, 1903), as given and modernized in the Internet Medieval Sourcebook:

"And indeed, we believe that you are a being than which nothing greater can be conceived. Or is there no such nature, since the fool has said in his heart, there is no God? (Psalms xiv. 1). But, at any rate, this very fool, when he hears of this being of which I speak, a being than which nothing greater can be conceived, understands what he hears, and what he understands is in his understanding; although he does not understand it to exist. For, it is one thing for an object to be in the understanding, and another to understand that the object exists."⁴

Anselm goes on to argue that "that, than which nothing greater can be conceived, cannot exist in the understanding alone," because if it existed only in the mind, it could be conceived to exist in reality as well, "which is greater."⁴ The argument moves from what the word "God" means to the existence of what it names. In the eighteenth century Kant would call this kind of argument "ontological," and the name has stuck. Aquinas's second objection is a close summary of it: "by this word is signified that thing than which nothing greater can be conceived," and "that which exists actually and mentally is greater than that which exists only mentally." One misprint in the Sourcebook's modernized text ("be" for "he") is corrected in the quotation above; the correction changes no sense.

The argument had been answered in Anselm's own lifetime by a monk named Gaunilo, whose reply, "In Behalf of the Fool," is printed as an appendix in the same Deane volume.⁴ Aquinas does not follow Gaunilo's line (Gaunilo's famous comparison was a perfect island). He makes his own distinction instead.

"On the contrary": the fool of the Psalm

"On the contrary, No one can mentally admit the opposite of what is self-evident; as the Philosopher (Metaph. iv, lect. vi) states concerning the first principles of demonstration. But the opposite of the proposition 'God is' can be mentally admitted: 'The fool said in his heart, There is no God' (Psalm 53:2). Therefore, that God exists is not self-evident."¹

The same verse of the Psalms that Anselm had used to set up his argument is here turned against it. Anselm read the fool as someone who contradicts himself; Aquinas reads him as evidence that the denial can at least be thought. The verse appears twice in the Psalter, as Psalm 14:1 and Psalm 53:1 in the King James Version ("The fool hath said in his heart, There is no God"); New Advent gives the reference as Psalm 53:2, counting the heading as a verse, and the Douay-Rheims, which follows the Latin numbering, has the same words in Psalms 13 and 52.

"I answer that": self-evident in itself, not to us

"I answer that, A thing can be self-evident in either of two ways: on the one hand, self-evident in itself, though not to us; on the other, self-evident in itself, and to us. A proposition is self-evident because the predicate is included in the essence of the subject, as 'Man is an animal,' for animal is contained in the essence of man. If, therefore the essence of the predicate and subject be known to all, the proposition will be self-evident to all; as is clear with regard to the first principles of demonstration, the terms of which are common things that no one is ignorant of, such as being and non-being, whole and part, and such like. If, however, there are some to whom the essence of the predicate and subject is unknown, the proposition will be self-evident in itself, but not to those who do not know the meaning of the predicate and subject of the proposition. Therefore, it happens, as Boethius says (Hebdom., the title of which is: 'Whether all that is, is good'), 'that there are some mental concepts self-evident only to the learned, as that incorporeal substances are not in space.' Therefore I say that this proposition, 'God exists,' of itself is self-evident, for the predicate is the same as the subject, because God is His own existence as will be hereafter shown (I:3:4). Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature, namely, by effects."¹

The answer turns on a distinction between what is known in itself and what is known to us. In God, Aquinas holds, essence and existence are the same: what God is and that God is are not two things. He argues for this in ST I, q. 3, a. 4, where one of his reasons runs: "whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence."⁵ So "God exists" is, strictly, a proposition whose predicate is contained in its subject. But no human being in this life knows God's essence. Human beings know God, in Aquinas's phrase, only "in a general and confused way," and so the proposition cannot be self-evident to us. It has to be reached by another road: from effects that are known to the cause that is not.

The replies

"Reply to Objection 1. To know that God exists in a general and confused way is implanted in us by nature, inasmuch as God is man's beatitude. For man naturally desires happiness, and what is naturally desired by man must be naturally known to him. This, however, is not to know absolutely that God exists; just as to know that someone is approaching is not the same as to know that Peter is approaching, even though it is Peter who is approaching; for many there are who imagine that man's perfect good which is happiness, consists in riches, and others in pleasures, and others in something else.

"Reply to Objection 2. Perhaps not everyone who hears this word 'God' understands it to signify something than which nothing greater can be thought, seeing that some have believed God to be a body. Yet, granted that everyone understands that by this word 'God' is signified something than which nothing greater can be thought, nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this precisely is not admitted by those who hold that God does not exist.

"Reply to Objection 3. The existence of truth in general is self-evident but the existence of a Primal Truth is not self-evident to us."¹

The reply to the first objection is gentle with John of Damascus. Aquinas grants that there is a natural knowledge of God of a sort: every human being wants happiness, and God is in fact the happiness the human heart wants. But wanting happiness is like seeing someone coming down the road without knowing who it is. Many people think the good they want lies in money or pleasure. The natural desire is real; the knowledge that the one desired is God is not given with it. Lesson 06 follows this thread into the questions on happiness in the Second Part, where Aquinas argues that human happiness lies in the vision of God.

The reply to the second objection makes two points against the Anselmian argument. The first is factual: not everyone understands "God" to mean "that than which nothing greater can be thought," since some ancient thinkers believed God was a body. The second is logical, and it is the one most philosophers have discussed. Even granting the definition, the argument shows only that the greatest conceivable being exists in the mind; it does not show that it exists in reality unless one already grants that such a being exists, and that is exactly what the atheist denies. Aquinas made the same two points at more length in the Summa contra Gentiles, book I, chapters 10 and 11, adding that "many ancients said that this world itself was God."⁶

Aquinas's verdict on the argument, then, is that it fails as a proof for human knowers, who do not know God's essence. He does not say that "God exists" is false, nor that it is not self-evident in itself: he says the opposite. Later Thomists and later defenders of Anselm have argued about whether he understood Anselm rightly. Some point out that the Proslogion is a prayer that begins from faith ("I believe in order to understand," Anselm writes in chapter 1), so that Anselm may never have meant to offer a proof to the unbeliever at all. That debate goes beyond the text of the Summa, which rejects the argument as it was stated to Aquinas, in the form given in objection 2.

Article 2: Can God's existence be demonstrated?

Having said that "God exists" is not self-evident to us, Aquinas must now show that it can be known some other way. A "demonstration" in the language of the medieval schools, taken from Aristotle's Posterior Analytics, is an argument that produces scientific knowledge (scientia): certain knowledge of a conclusion through its causes or principles. The article asks whether such an argument is possible about God.

The objections: faith, essence and proportion

"Objection 1. It seems that the existence of God cannot be demonstrated. For it is an article of faith that God exists. But what is of faith cannot be demonstrated, because a demonstration produces scientific knowledge; whereas faith is of the unseen (Hebrews 11:1). Therefore it cannot be demonstrated that God exists.

"Objection 2. Further, the essence is the middle term of demonstration. But we cannot know in what God's essence consists, but solely in what it does not consist; as Damascene says (De Fide Orth. i, 4). Therefore we cannot demonstrate that God exists.

"Objection 3. Further, if the existence of God were demonstrated, this could only be from His effects. But His effects are not proportionate to Him, since He is infinite and His effects are finite; and between the finite and infinite there is no proportion. Therefore, since a cause cannot be demonstrated by an effect not proportionate to it, it seems that the existence of God cannot be demonstrated."¹

These are serious objections, and the first one is the objection a believer would raise. If God's existence can be proved, why does the Creed say "I believe in one God"? Would a proof not make faith unnecessary?

"On the contrary": Romans 1:20

"On the contrary, The Apostle says: 'The invisible things of Him are clearly seen, being understood by the things that are made' (Romans 1:20). But this would not be unless the existence of God could be demonstrated through the things that are made; for the first thing we must know of anything is whether it exists."¹

The authority for the whole enterprise of the Five Ways is a verse of Scripture. Paul's sentence in Romans 1:20 (in the King James Version, "For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse") is the text that Catholic and Protestant writers alike have argued over whenever natural knowledge of God is discussed. Calvin, as will be seen below, read the same verse to quite different effect.

"I answer that": from effects to cause

"I answer that, Demonstration can be made in two ways: One is through the cause, and is called 'a priori,' and this is to argue from what is prior absolutely. The other is through the effect, and is called a demonstration 'a posteriori'; this is to argue from what is prior relatively only to us. When an effect is better known to us than its cause, from the effect we proceed to the knowledge of the cause. And from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us; because since every effect depends upon its cause, if the effect exists, the cause must pre-exist. Hence the existence of God, in so far as it is not self-evident to us, can be demonstrated from those of His effects which are known to us."¹

This is the method of all five arguments that follow. None of them reasons from what God is to the fact that God exists (that would be "a priori," and article 1 has just ruled it out for human knowers). Every one reasons from something in the world that the senses can observe, an effect, back to a cause that must exist if the effect does. The same principle is stated again in ST I, q. 12, a. 12, where Aquinas asks whether God can be known in this life by natural reason: "Our natural knowledge begins from sense. Hence our natural knowledge can go as far as it can be led by sensible things. But our mind cannot be led by sense so far as to see the essence of God; because the sensible effects of God do not equal the power of God as their cause." From those effects, he goes on, "we can be led from them so far as to know of God 'whether He exists.'"⁷

The replies: "preambles to the articles"

"Reply to Objection 1. The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for faith presupposes natural knowledge, even as grace presupposes nature, and perfection supposes something that can be perfected. Nevertheless, there is nothing to prevent a man, who cannot grasp a proof, accepting, as a matter of faith, something which in itself is capable of being scientifically known and demonstrated.

"Reply to Objection 2. When the existence of a cause is demonstrated from an effect, this effect takes the place of the definition of the cause in proof of the cause's existence. This is especially the case in regard to God, because, in order to prove the existence of anything, it is necessary to accept as a middle term the meaning of the word, and not its essence, for the question of its essence follows on the question of its existence. Now the names given to God are derived from His effects; consequently, in demonstrating the existence of God from His effects, we may take for the middle term the meaning of the word 'God'.

"Reply to Objection 3. From effects not proportionate to the cause no perfect knowledge of that cause can be obtained. Yet from every effect the existence of the cause can be clearly demonstrated, and so we can demonstrate the existence of God from His effects; though from them we cannot perfectly know God as He is in His essence."¹

The phrase "preambles to the articles" (praeambula ad articulos) became the standard term in Catholic theology for truths such as God's existence that faith assumes and reason can reach. The reply to the first objection also contains a sentence that is easy to miss: a person who "cannot grasp a proof" may accept by faith what could in principle be proved. Aquinas does not hold that anyone must work through the Five Ways before believing in God. He holds the reverse, and he says why in the Second Part of the Second Part, where he asks "Whether it is necessary to believe those things which can be proved by natural reason?" His answer gives three reasons:

"I answer that, It is necessary for man to accept by faith not only things which are above reason, but also those which can be known by reason: and this for three motives. First, in order that man may arrive more quickly at the knowledge of Divine truth. Because the science to whose province it belongs to prove the existence of God, is the last of all to offer itself to human research, since it presupposes many other sciences: so that it would not by until late in life that man would arrive at the knowledge of God. The second reason is, in order that the knowledge of God may be more general. For many are unable to make progress in the study of science, either through dullness of mind, or through having a number of occupations, and temporal needs, or even through laziness in learning, all of whom would be altogether deprived of the knowledge of God, unless Divine things were brought to their knowledge under the guise of faith. The third reason is for the sake of certitude. For human reason is very deficient in things concerning God. A sign of this is that philosophers in their researches, by natural investigation, into human affairs, have fallen into many errors, and have disagreed among themselves."⁸

("Would not by until late in life" is New Advent's text; the sense is "would not be until late in life.") The reasons are practical and modest: proofs take time, most people have neither the leisure nor the training, and reason makes mistakes. In the first question of the Summa Aquinas had already stated the general rule that governs all of this: "Since therefore grace does not destroy nature but perfects it, natural reason should minister to faith as the natural bent of the will ministers to charity" (ST I, q. 1, a. 8, ad 2).⁹ Lesson 07 takes up the relation of nature and grace in Aquinas's treatise on grace, which is where Protestant and Catholic readers part company most sharply.

Article 3: "Whether God exists?"

The third article is short, a little over a thousand words in English, and it has had more readers than any other page Aquinas wrote. Its form is unusual in one respect: it has only two objections, and the body of the article is longer than the rest put together.

The two objections: evil, and nature without God

"Objection 1. It seems that God does not exist; because if one of two contraries be infinite, the other would be altogether destroyed. But the word 'God' means that He is infinite goodness. If, therefore, God existed, there would be no evil discoverable; but there is evil in the world. Therefore God does not exist.

"Objection 2. Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God's existence."¹

These two objections are still the two main reasons people give for not believing in God: the existence of evil, and the sufficiency of natural explanations. The second objection is a medieval form of the rule later called "Ockham's razor" (do not multiply causes beyond need), named after the English friar William of Ockham, who was born some years after Aquinas died. Readers who think of the Five Ways as a medieval relic may be surprised to find that Aquinas raised exactly these objections against his own conclusion.

"On the contrary": "I am Who am"

"On the contrary, It is said in the person of God: 'I am Who am.' (Exodus 3:14)"¹

The "On the contrary" of the most famous philosophical argument in the Summa is a verse of Scripture: God's answer to Moses at the burning bush, in the Latin Vulgate's words Ego sum qui sum (the Douay-Rheims "I AM WHO AM"; the King James Version "I AM THAT I AM"). This is the authority Aquinas sets against the objections, before any argument is given. The Latin Vulgate (translated about 382 to 405 at the request of Pope Damasus) was the Bible Aquinas used; lesson 04 of The History of the Bible describes it. Later, in ST I, q. 13, a. 11, Aquinas will argue that "He Who Is" is the most proper name of God, because God's essence is His very existence.¹⁰

The sentence that introduces the Five Ways

"I answer that, The existence of God can be proved in five ways."¹

The Latin is Respondeo dicendum quod Deum esse quinque viis probari potest.² The word "ways" (viae) is Aquinas's own, and it gave the arguments their name. Each way is a single paragraph. Each follows the same plan: a fact that the senses show, a principle about causes, a step that rules out an infinite series of a certain kind, and a conclusion that there is a first something, followed by a short phrase that ties that first something to the word "God." The chart below sets them side by side.

The Five Ways one by one

The First Way: from motion

"The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God."¹

What "motion" means. The Latin word is motus, and for Aquinas, following Aristotle, it covers every kind of change: a thing moving from place to place, but also water growing hot, a plant growing, a leaf turning color. The definition is given in the text: motion "is nothing else than the reduction of something from potentiality to actuality." Cold wood can become hot; it is "potentially" hot. When fire heats it, the wood's potential is made actual. The pair of terms potentia and actus (potency and act) comes from Aristotle and runs through the whole of Aquinas's thought.

The steps. The argument has four. (1) Some things are changing; the senses show it. (2) Whatever changes is changed by something else, because a potential cannot actualize itself: what is only potentially hot cannot make itself actually hot, since to do so it would already have to be actually hot. (3) The series of things changed by others cannot go on to infinity, because in this kind of series the later members move "only inasmuch as they are put in motion by the first mover," as a staff moves only because a hand moves it. (4) So there is a first mover that is not moved by any other.

The kind of series. The example of the staff and the hand is the key to the third step. A staff pushing a stone has no power of its own to push; it passes on, at this moment, the motion it is receiving from the hand at this moment. If the hand stops, the staff stops. A chain of such movers, however long, is like a train of carriages without an engine: adding carriages does not supply the pull. The series the First Way has in mind is a series of movers acting all at once, each depending here and now on the one before it. This point is treated at length in the section on causal series below, because almost every popular objection to the Five Ways turns on it.

The Aristotle behind it. The example of the stick and the hand comes from Aristotle's Physics, book VIII, chapter 5, which in the Oxford translation of R. P. Hardie and R. K. Gaye reads: "the stick moves the stone and is moved by the hand, which again is moved by the man: in the man, however, we have reached a movent that is not so in virtue of being moved by something else."¹¹ In Metaphysics XII, chapter 7, Aristotle concludes that "there is something which moves without being moved, being eternal, substance, and actuality."¹² Aquinas says plainly in the Summa contra Gentiles that the argument from motion is Aristotle's: "We shall first set forth the arguments by which Aristotle proceeds to prove that God exists. The aim of Aristotle is to do this in two ways, beginning with motion."⁶ Lesson 03 explains why Aquinas calls Aristotle simply "the Philosopher."

Aristotle's unmoved mover and the God of the Bible are not the same idea, and Aquinas knew it. Aristotle's first mover moves the heavens eternally, as an object of desire and thought; it does not create, does not know the world in detail, and is one of several unmoved movers in book XII. Aquinas's First Way borrows the argument but not the whole of Aristotle's picture, and the remaining questions of the treatise on God (on God's simplicity, perfection, goodness, infinity, knowledge, will and providence, ST I, qq. 3 to 26) are where he argues that the first mover has the attributes the Christian faith ascribes to God.

The Second Way: from efficient causes

"The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God."¹

An "efficient cause" in Aristotle's scheme of four causes is the agent that brings something about: the sculptor of a statue, the parent of a child, the fire that heats the pot. (The other three are the material cause, the formal cause and the final cause, the purpose.) Where the First Way starts from things being changed, the Second Way starts from things being brought about, and in particular from an "order" of causes: a cause that is itself caused, which is caused by something else.

The logic is short. Nothing causes itself, because it would have to exist before it existed. In an ordered series, the first causes the middle and the middle causes the last. Take away the first, and the middle and the last are taken away too. An infinite series would have no first member; therefore it would have no middle or last members either; but there plainly are such caused causes; therefore there is a first efficient cause. Here too Aquinas tells his reader in the Summa contra Gentiles where the argument comes from: "In Metaphysics II Aristotle also uses another argument to show that there is no infinite regress in efficient causes and that we must reach one first cause, God."⁶ In that book of the Metaphysics Aristotle writes, in W. D. Ross's translation, "evidently there is a first principle, and the causes of things are neither an infinite series nor infinitely various in kind," and he gives an example of a series of movers: "man for instance being acted on by air, air by the sun, the sun by Strife, and so on without limit."¹³

Readers sometimes ask how the Second Way differs from the First. The usual Thomist answer is that the First Way concerns the change of things that already exist, while the Second concerns the causing of things in being: a thing's coming to be and continuing to be. Both use the same kind of ordered series, in which a middle cause causes only while it is being caused.

The Third Way: from possibility and necessity

"The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence, which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God."¹

Possible and necessary. A "possible" being, in this argument, is a thing that comes into being and passes away: a plant, an animal, a human being, anything "generated" and subject to "corruption" (decay). A "necessary" being is one that does not come into being or pass away by nature. In medieval cosmology the heavenly bodies, the angels and human souls were thought to be necessary in this sense: they had no natural tendency to decay, though they had been created. So the argument has two halves. The first half argues that not everything can be of the kind that comes and goes. The second half argues that if there are necessary beings that owe their necessity to another, there must be one that has its necessity of itself.

The disputed step. The sentence "if everything is possible not to be, then at one time there could have been nothing in existence" has drawn more criticism than any other in the Five Ways. Read one way, it seems to argue from "each thing at some time does not exist" to "at some time nothing exists," which does not follow (each runner in a relay can be off the track at some time without the track ever being empty). The Oxford philosopher Anthony Kenny, in The Five Ways: St. Thomas Aquinas' Proofs of God's Existence (1969), was among those who pressed this objection.¹⁴ Defenders answer in various ways. Some point out that the Latin of the key clause (quod possibile est non esse, quandoque non est, "that which is possible not to be at some time is not") assumes, with Aristotle, an infinite past time, in which every unrealized possibility would have been realized; others, Edward Feser among them, read the argument chiefly through its second half, as a claim that whatever does not have its existence of itself must receive it from something that does.¹⁵ The scholarly debate is long and unsettled, and readers should know that Thomists themselves disagree about how this paragraph is best read.

Its sources. Aquinas does not name a source for the Third Way in this article. Modern historians of medieval philosophy have pointed to arguments from possibility and necessity in the Muslim philosopher Avicenna (Ibn Sina, died 1037) and in the Jewish philosopher Moses Maimonides (died 1204), whose Guide for the Perplexed Aquinas knew and cites elsewhere in the Summa as "Rabbi Moses." Lesson 03 counts how often each of these authorities appears in the Summa.

The Fourth Way: from the grades of being

"The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But 'more' and 'less' are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God."¹

The Fourth Way is the one modern readers find strangest, and the one most shaped by Plato as well as Aristotle. It starts from the fact that people rank things: one act is better than another, one thing is "nobler" than another. It argues that such comparisons are made by reference to a highest of the kind, and that the highest in a kind is the cause of the rest of that kind. The example of fire and heat comes straight from Metaphysics II, chapter 1, cited in the article as "Metaph. ii": in Ross's translation, "a thing has a quality in a higher degree than other things if in virtue of it the similar quality belongs to the other things as well (e.g. fire is the hottest of things; for it is the cause of the heat of all other things); so that that causes derivative truths to be true is most true."¹³ In the Summa contra Gentiles Aquinas again says, "Another argument may also be gathered from the words of Aristotle."⁶

The fire example belongs to an older physics, in which fire was the element that was hot by nature. Critics such as Kenny judged that the Fourth Way depends on a Platonic theory of degrees of being that few modern philosophers accept.¹⁴ Thomist readers reply that the example is only an illustration, and that the argument concerns qualities such as goodness and being that have no upper limit in the way that heat does; on that reading the Fourth Way is an argument about "participation," the idea that limited things have goodness and being only by sharing in a source that has them without limit. Of the five, this way leads most directly into the next questions of the Summa, on God's perfection (q. 4) and goodness (q. 6).

The Fifth Way: from the governance of the world

"The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God."¹

The Fifth Way is often called the "design argument," and in popular books it is often merged with the argument of the English clergyman William Paley, whose Natural Theology (1802) compared the world to a watch found on a heath, whose parts are so fitted together that they must have had a maker.¹⁶ The two arguments are different. Paley started from complex arrangements of parts (the eye, the watch) and argued to a designer who put them together. Aquinas starts from something simpler and more general: that natural things, even those without minds, "act for an end," acting "always, or nearly always, in the same way." An acorn tends to become an oak and not a frog; fire tends to heat and not to cool; a stone falls. That regular tending toward a definite result is what Aquinas means by acting "for an end," and he argues that a thing without knowledge can only tend toward a goal if a mind directs it, as an arrow flies to the target because an archer aimed it.

The difference matters for the modern discussion. Paley's argument was widely thought to have been answered by Charles Darwin's account of natural selection (1859), which offered an explanation of complex adaptations without a designer of each one. The Fifth Way does not depend on any particular complex adaptation; it would apply to a world of falling stones and burning fires with no living things in it at all. Whether that makes it stronger or weaker is debated, but the two arguments should not be confused. Aquinas, in the Summa contra Gentiles, gives this argument's ancestry: "Damascene proposes another argument for the same conclusion taken from the government of the world. Averroes likewise hints at it."⁶

"This all men speak of as God": the five closing phrases

Each way ends with a short phrase that has been read in very different ways. The Latin and English are:

Way	Latin (Leonine text)	English (1920 translation)
First	et hoc omnes intelligunt Deum	"and this everyone understands to be God"
Second	quam omnes Deum nominant	"to which everyone gives the name of God"
Third	quod omnes dicunt Deum	"This all men speak of as God"
Fourth	et hoc dicimus Deum	"and this we call God"
Fifth	et hoc dicimus Deum	"and this being we call God"

Sources: Corpus Thomisticum, ST I, q. 2, a. 3;² New Advent, ST I, q. 2, a. 3.¹

Critics have read these phrases as a sleight of hand: an argument proves a first mover, and then Aquinas simply announces that it is God. Thomist readers answer that the phrases make a modest linguistic claim, that this is what people mean by "God," and that the claim to have shown the first cause to be one, simple, good, infinite, knowing and willing is made only in the questions that follow. Article 2, reply 2 had already prepared for this: in proving that God exists, "we may take for the middle term the meaning of the word 'God'," because "the names given to God are derived from His effects."¹ In other words the Five Ways prove that there is a first mover, a first cause, a necessary being, a maximum and a governing intelligence; that each of these is the one God of the Creed is the work of ST I, qq. 3 to 26. Brian Davies, among others, has stressed that Aquinas does not think of God as one more item in the universe, however large, so that the "first" in "first mover" means first in dependence, not first in a line of things like itself.¹⁷

The replies: evil, and nature

"Reply to Objection 1. As Augustine says (Enchiridion xi): 'Since God is the highest good, He would not allow any evil to exist in His works, unless His omnipotence and goodness were such as to bring good even out of evil.' This is part of the infinite goodness of God, that He should allow evil to exist, and out of it produce good.

"Reply to Objection 2. Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article."¹

The reply on evil is short: a quotation from Augustine's Enchiridion and a single sentence. Aquinas returns to evil at length elsewhere (ST I, qq. 48 and 49), where he argues that evil is a lack of a good that ought to be present and not a thing in its own right. The reply on nature makes clear that the Five Ways do not compete with natural explanations. Nature really does work, and human will really does choose; Aquinas's claim is that nature and will, being "changeable and capable of defect," are themselves caused, and must be traced to a first cause that is not.

Two kinds of causal series: the most common misreading

The single most common misreading of the Five Ways is to take them as arguments that the universe must have had a beginning in time: trace causes back through history, the reasoning goes, and since the series cannot be infinite, there must have been a first event, and God caused it. That argument has a long history of its own (it was developed by the Muslim theologians of the kalam and argued by some Christian writers of Aquinas's own century; it is now called the kalam argument, and modern defenders include the American philosopher William Lane Craig), but it is not the argument of ST I, q. 2, a. 3, and Aquinas said in so many words that it cannot be proved by reason.

Aquinas: the beginning of the world cannot be demonstrated

In ST I, q. 46, a. 2, "Whether it is an article of faith that the world began?", Aquinas lists eight arguments that the world's beginning can be proved, some of them from Christian writers of his own day, and then answers:

"I answer that, By faith alone do we hold, and by no demonstration can it be proved, that the world did not always exist, as was said above of the mystery of the Trinity (I:32:1). The reason of this is that the newness of the world cannot be demonstrated on the part of the world itself. For the principle of demonstration is the essence of a thing. Now everything according to its species is abstracted from 'here' and 'now'; whence it is said that universals are everywhere and always. Hence it cannot be demonstrated that man, or heaven, or a stone were not always. Likewise neither can it be demonstrated on the part of the efficient cause, which acts by will. For the will of God cannot be investigated by reason, except as regards those things which God must will of necessity; and what He wills about creatures is not among these, as was said above (I:19:3). But the divine will can be manifested by revelation, on which faith rests. Hence that the world began to exist is an object of faith, but not of demonstration or science. And it is useful to consider this, lest anyone, presuming to demonstrate what is of faith, should bring forward reasons that are not cogent, so as to give occasion to unbelievers to laugh, thinking that on such grounds we believe things that are of faith."¹⁸

Aquinas believed that the world had a beginning; he held it as an article of faith, from Genesis 1:1 and the Creed. What he denied is that reason can prove it. The last sentence of the passage is a warning that he took seriously: bad arguments for a true belief do harm, because they lead unbelievers to think that the belief rests on them. (The phrase "By faith alone" here, sola fide in the Latin, has nothing to do with the Reformation debate over justification; it means that the world's beginning is known only from revelation. Lesson 07 discusses another place where Aquinas uses the words sola fide.)

The same point appears in the Summa contra Gentiles. After setting out Aristotle's arguments from motion, Aquinas notes that they "presuppose the eternity of motion, which Catholics consider to be false," and replies: "The most efficacious way to prove that God exists is on the supposition that the world is eternal. Granted this supposition, that God exists is less manifest."⁶ In other words, Aquinas deliberately framed his arguments so that they would work even if the world had always existed, as Aristotle believed. If the world began, the case for a cause is easier; he wanted an argument that did not need that premise.

Per se and accidental series: Aquinas's own distinction

How can the Five Ways deny an infinite series of causes while q. 46 allows that an infinite series of past causes cannot be ruled out by reason? Aquinas answers this question himself, in his reply to the seventh objection of q. 46, a. 2. The objection had argued that if the world were eternal, there would be an infinite series of fathers and sons, and therefore an infinite series of efficient causes, which Aristotle forbids. Aquinas replies:

"Reply to Objection 7. In efficient causes it is impossible to proceed to infinity per se, thus, there cannot be an infinite number of causes that are per se required for a certain effect; for instance, that a stone be moved by a stick, the stick by the hand, and so on to infinity. But it is not impossible to proceed to infinity 'accidentally' as regards efficient causes; for instance, if all the causes thus infinitely multiplied should have the order of only one cause, their multiplication being accidental, as an artificer acts by means of many hammers accidentally, because one after the other may be broken. It is accidental, therefore, that one particular hammer acts after the action of another; and likewise it is accidental to this particular man as generator to be generated by another man; for he generates as a man, and not as the son of another man. For all men generating hold one grade in efficient causes, viz. the grade of a particular generator. Hence it is not impossible for a man to be generated by man to infinity; but such a thing would be impossible if the generation of this man depended upon this man, and on an elementary body, and on the sun, and so on to infinity."¹⁸

This reply is the key to the Five Ways. There are two kinds of series.

In a series ordered per se ("of itself"), each member causes only because, and only while, the member before it is causing. The stone moves because the stick moves it, the stick because the hand moves it, the hand because the man moves it, all at the same moment. Take away any earlier member and every later member stops at once. The middle members have no causal power of their own to contribute; they pass on what they receive. A series of this kind, Aquinas says, cannot be infinite, because without a first member nothing would be passed on at all.

In a series ordered accidentally ("by accident," meaning not by the nature of the causing), each member does its causing on its own once it exists. A father begets a son "as a man, and not as the son of another man": his own father may be long dead, and the son is still begotten. A carpenter's first hammer may have broken years ago; the carpenter goes on driving nails with the tenth. Such a series is strung out in time, and nothing in reason, Aquinas says, forbids it from going back without end.

The Five Ways argue about the first kind of series. The staff and the hand in the First Way, and "in all efficient causes following in order, the first is the cause of the intermediate cause" in the Second, describe a series of causes acting together, here and now. The question the Five Ways ask is what keeps the world in motion and in being now, and the answer is a first cause on which everything depends at every moment, not a first event at the start of history.

The last clause of the reply shows what Aquinas had in mind for the per se series that runs through nature: the generation of a man depends here and now "upon this man, and on an elementary body, and on the sun, and so on." In medieval physics the heavenly bodies were thought to be the active causes of generation on earth. The physics has changed; whether the argument survives the change is one of the questions discussed below. Modern Thomists offer other examples of per se series, such as the dependence of a living thing at each moment on the chemical and physical conditions that sustain it.

"Then what caused God?"

The popular objection "If everything has a cause, what caused God?" was put most famously by the British philosopher Bertrand Russell in his 1927 lecture "Why I Am Not a Christian," where he recalled that as a young man he had been persuaded by the question "Who made God?" in John Stuart Mill's Autobiography.¹⁹ Whatever its force against other arguments, the objection does not touch the text of the Five Ways, because none of them says that everything has a cause. The First Way says that whatever is in motion (passing from potency to act) is moved by another. The Second says that in the world of sense there is an order of caused causes. The Third says that things that come and go need a cause, and that necessary things that have their necessity "caused by another" do too. Each argument concludes to something that is not in the class it began from: a mover that is not moved, a cause that is not caused, a being that has "of itself its own necessity." Asking what caused it is asking what moved the unmoved mover. Critics may argue that such a being is impossible or meaningless (Hume did, as will be seen), but that is a different objection.

The Five Ways do not end at question 2

A second common misreading is to treat article 3 as Aquinas's whole account of God, as though he thought a first mover proved the God of Abraham, Isaac and Jacob. The very next question begins: "When the existence of a thing has been ascertained there remains the further question of the manner of its existence, in order that we may know its essence." That prologue to question 3, as printed in the CCEL text of the same translation, goes on to say: "Now, because we cannot know what God is, but rather what He is not, we have no means for considering how God is, but rather how He is not."²⁰ What follows, in questions 3 to 11, is the "way of remotion" or negative theology: God is not a body, not composed, not limited, not changing, not in time. Only in later questions does Aquinas argue to God's knowledge, will, love, justice, mercy and providence (qq. 14 to 24), and the Trinity is not reached until question 27, because, as q. 46 has just said and q. 32, a. 1 argues at length, the Trinity cannot be known by reason at all. The Five Ways are the first step of a long climb, and Aquinas says at the climb's start that it can go only part of the way.

Objections from Hume and Kant

Neither David Hume (1711 to 1776) nor Immanuel Kant (1724 to 1804) wrote a commentary on ST I, q. 2. Their objections were aimed at the arguments for God's existence as they knew them in the eighteenth century, chiefly in the forms given by Samuel Clarke, Gottfried Wilhelm Leibniz, Christian Wolff and the British writers on design. But their objections have been applied to the Five Ways ever since, and no account of the Five Ways today can leave them out. Both are quoted here in their own words.

Hume: the Dialogues Concerning Natural Religion

Hume's Dialogues Concerning Natural Religion were published in 1779, three years after his death. The book is a conversation among three characters: Cleanthes, who defends an argument from design; Demea, who defends an argument from the necessity of a first cause; and Philo, a skeptic. Scholars still argue over which character speaks for Hume. In Part IX, after Demea has set out the argument that a chain of causes needs a necessary first cause, Cleanthes answers him with two objections that are now standard.

The first is that "necessary existence" has no clear meaning, and that if anything is necessary, it might as well be the material world: "why may not the material universe be the necessarily existent Being, according to this pretended explication of necessity? We dare not affirm that we know all the qualities of matter; and for aught we can determine, it may contain some qualities, which, were they known, would make its non-existence appear as great a contradiction as that twice two is five."²¹

The second is that a chain of causes needs no cause beyond its members:

"In such a chain, too, or succession of objects, each part is caused by that which preceded it, and causes that which succeeds it. Where then is the difficulty? But the whole, you say, wants a cause. I answer, that the uniting of these parts into a whole, like the uniting of several distinct countries into one kingdom, or several distinct members into one body, is performed merely by an arbitrary act of the mind, and has no influence on the nature of things. Did I show you the particular causes of each individual in a collection of twenty particles of matter, I should think it very unreasonable, should you afterwards ask me, what was the cause of the whole twenty. This is sufficiently explained in explaining the cause of the parts."²¹

Against the argument from design, Philo in Part V presses the point that the world, judged by experience alone, gives no grounds for inferring one perfect maker rather than many imperfect ones: "This world, for aught he knows, is very faulty and imperfect, compared to a superior standard; and was only the first rude essay of some infant deity, who afterwards abandoned it, ashamed of his lame performance."²¹ Elsewhere in the book Philo argues that human beings have experience of houses and watches being made, but none of worlds being made, so the analogy from human design to divine design is weak.

Kant: the Critique of Pure Reason

Kant's Critique of Pure Reason (first edition 1781, second 1787) contains, in its section on "the Ideal of Pure Reason," the most influential modern attack on all three types of argument for God's existence, which Kant named the ontological, the cosmological and the physico-theological (design). The quotations here are from J. M. D. Meiklejohn's English translation as given by Project Gutenberg.

Against the ontological argument, the one Aquinas had already rejected in article 1, Kant made his famous point about existence: "Being is evidently not a real predicate, that is, a conception of something which is added to the conception of some other thing. It is merely the positing of a thing, or of certain determinations in it."²² Saying that a thing exists adds nothing to the concept of the thing; so no concept, however great, can contain its own existence.

Against the cosmological argument, which is the family the first three of the Five Ways belong to, Kant made two charges. The first was that the principle of causality has meaning only within experience. Among the "fallacies" he found in the argument was "the transcendental principle: 'Everything that is contingent must have a cause', a principle without significance, except in the sensuous world."²² The second was that the cosmological and design arguments secretly depend on the ontological argument, which he had already rejected: "Thus the physico-theological is based upon the cosmological, and this upon the ontological proof of the existence of a Supreme Being; and as besides these three there is no other path open to speculative reason, the ontological proof, on the ground of pure conceptions of reason, is the only possible one, if any proof of a proposition so far transcending the empirical exercise of the understanding is possible at all."²² Kant's reasoning was that the cosmological argument proves at most a "necessary being," and then has to identify that being with the "most real being" (God) by an argument from concepts alone, which is the ontological argument in disguise.

Kant did not conclude that God does not exist. He argued that theoretical reason can neither prove nor disprove God's existence, and that belief in God is instead a postulate of practical (moral) reason. His preface to the second edition (1787) contains the much-quoted sentence, in Meiklejohn's words, "I must, therefore, abolish knowledge, to make room for belief"; other translators render it "deny knowledge in order to make room for faith."²²

What modern Thomists reply

The replies of modern Thomist philosophers to Hume and Kant are many, and this section reports a few of the best known, attributed to those who make them. They are summaries of positions, not quotations, since the books are under copyright.

On Hume's "whole twenty." Edward Feser, an American Catholic philosopher, in Aquinas (Oneworld, 2009) and Five Proofs of the Existence of God (Ignatius Press, 2017), replies that Hume's chain is a series in time, each member caused by the one before it, the very kind of series Aquinas said in ST I, q. 46, a. 2, ad 7 cannot be ruled out. Aquinas's First and Second Ways concern a series of causes acting together, in which the middle members have no causal power of their own, so explaining each member by the one before it explains nothing until a first member is reached.¹⁵ On this reading the twenty particles of matter are not like the Five Ways' staff and hand at all.

On "why not the universe?" Thomists answer that the universe, made up of things that change and come and go, has exactly the features (potency, composition, dependence) that the Five Ways argue need a cause; a "necessary being" in Aquinas's sense must be pure act, with no unrealized potential, which a changing material world is not. This is why, they argue, the Five Ways cannot be judged apart from question 3 on God's simplicity.¹⁵

On Kant's charge that the cosmological argument depends on the ontological. Thomists reply that Aquinas never argues from the concept of a necessary being to its existence: he argues from observed effects (things in motion, things caused, things that come and go) to the existence of their cause, and he identifies the cause with God only afterwards, by further arguments about what such a cause must be like. The Thomist philosopher Gaven Kerr, in Aquinas's Way to God: The Proof in De Ente et Essentia (Oxford University Press, 2015), defends a related argument from Aquinas's early work On Being and Essence, which reasons from the distinction between what a thing is and the fact that it exists to a first cause whose essence is its existence.²³ Kerr's book is a reminder that the Five Ways are not Aquinas's only argument for God's existence, and some Thomists hold that the argument from essence and existence is the deeper one beneath them all.

On Kant's limit of causality to experience. Thomists reply that Kant's limit depends on his own theory of knowledge, in which causality is a form the mind imposes on experience; if that theory is rejected, as Aquinas's realism rejects it, the objection loses its force. This is a disagreement about the starting points of philosophy, and it has not been settled.

On God as "one thing among others." Brian Davies, an English Dominican philosopher at Fordham University, in The Thought of Thomas Aquinas (Oxford, 1992) and Thomas Aquinas's Summa Theologiae: A Guide and Commentary (Oxford, 2014), argues that many objections assume that "God" names a very large and powerful individual within the world, a being among beings, and that Aquinas's God, whose essence is to exist, is not that kind of thing at all. Davies also stresses that ST I, q. 2 sits inside a work of theology written for students who already believed, so that it should not be read chiefly as an appeal to atheists.¹⁷ Fergus Kerr, a Scottish Dominican, in After Aquinas: Versions of Thomism (Blackwell, 2002), surveys how differently the Five Ways have been read by twentieth-century Thomists themselves, from those who treat them as strict proofs to those who treat them as summaries of the way a believer already sees the world.²⁴

On Aristotle's physics. Critics since the seventeenth century have argued that the First Way depends on a physics of motion that Galileo and Newton overturned, since Newton's first law says a body keeps moving without any mover. Kenny pressed this point in 1969.¹⁴ Thomists reply that the First Way concerns change in the broad sense (potency becoming act), and that a body moving at constant speed in a straight line is not, in Aristotle's sense, undergoing change in the relevant respect; the argument, they say, applies to any change in which something gains a property it did not have. Readers will find both sides argued at length in the books named here.

Catholic teaching on knowing God by reason

Catholic readers will want to know what weight the Church gives to the Five Ways. No council has defined the Five Ways themselves, or required Catholics to accept any one of them. What the Church has taught is the broader claim of ST I, q. 2, a. 2: that God can be known by reason from created things. The First Vatican Council, in its Dogmatic Constitution Dei Filius (session 3, April 24, 1870), chapter 2, teaches that "God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason," and quotes Romans 1:20, the same verse Aquinas used as his "On the contrary." Its canon on the point reads: "If anyone says that the one, true God, our creator and lord, cannot be known with certainty from the things that have been made, by the natural light of human reason: let him be anathema."²⁵ The council says "known with certainty"; it does not use the word "demonstrated."

The stronger word appears forty years later in the Oath against Modernism, which Pope Pius X issued in 1910, to be sworn by "all clergy, pastors, confessors, preachers, religious superiors, and professors in philosophical-theological seminaries": "I profess that God, the origin and end of all things, can be known with certainty by the natural light of reason from the created world (see Rom. 1:19), that is, from the visible works of creation, as a cause from its effects, and that, therefore, his existence can also be demonstrated."²⁶ The phrase "as a cause from its effects" is very close to Aquinas's "from effects to cause" in ST I, q. 2, a. 2. (The oath ceased to be required in 1967.) Leo XIII's encyclical Aeterni Patris (1879), which made Aquinas the model for Catholic philosophy, is covered in lesson 02 and is not repeated here.

What Protestant writers have said

Protestant readers have taken the Five Ways in several directions, and the differences run within Protestantism as much as between Protestants and Catholics. The positions below are stated, as far as possible, in their authors' own words.

Calvin: the sense of deity and the book of creation

John Calvin's Institutes of the Christian Religion (first edition 1536, final Latin edition 1559) begins its account of the knowledge of God with a claim that sounds at first like the objection of ST I, q. 2, a. 1. Book I, chapter 3, section 1, in Henry Beveridge's translation (1845):

"That there exists in the human minds and indeed by natural instinct, some sense of Deity, we hold to be beyond dispute, since God himself, to prevent any man from pretending ignorance, has endued all men with some idea of his Godhead, the memory of which he constantly renews and occasionally enlarges, that all to a man being aware that there is a God, and that he is their Maker, may be condemned by their own conscience when they neither worship him nor consecrate their lives to his service."²⁷

Calvin's "sense of Deity" (sensus divinitatis) is close to the "general and confused" knowledge that Aquinas grants in his reply to the first objection of article 1, though Calvin stresses that its purpose is to leave human beings without excuse. In chapter 5 Calvin turns to the knowledge of God from the world, and here he is as confident as Aquinas that creation shows its maker: God has chosen "so to manifest his perfections in the whole structure of the universe, and daily place himself in our view, that we cannot open our eyes without being compelled to behold him."²⁸ But at the end of the same chapter (section 14) he draws a conclusion that Aquinas does not:

"In vain for us, therefore, does Creation exhibit so many bright lamps lighted up to show forth the glory of its Author. Though they beam upon us from every quarter, they are altogether insufficient of themselves to lead us into the right path. Some sparks, undoubtedly, they do throw out; but these are quenched before they can give forth a brighter effulgence."²⁸

For Calvin, the fault lies in the human being who reads the evidence: sin has so darkened the mind that the knowledge of God from nature, though real and enough to condemn, does not lead to true knowledge of God without Scripture, which Calvin compares in the next chapter to spectacles that let weak eyes read. Aquinas, for his part, also held that human reason is "very deficient in things concerning God" (II-II, q. 2, a. 4, quoted above) and that most people learn of God by faith. The two agree that God's existence is shown in creation and that Scripture is needed; they differ on how much fallen reason can do with what creation shows.

The Reformed scholastics

Protestant theologians of the late sixteenth and seventeenth centuries, the "Reformed scholastics," taught natural theology in the schools and used arguments drawn from Aquinas. The Reformed historian and theologian J. V. Fesko, writing in Credo Magazine in 2023, names Francis Turretin, Franciscus Junius, Gulielmus Bucanus and others among Reformed writers who drew on Aquinas's proofs for God's existence, and argues that this was the mainstream of Reformed theology before the twentieth century.²⁹ His fuller case is made in his book Reforming Apologetics (Baker Academic, 2019).

Van Til: the presuppositional critique

The sharpest Protestant criticism of the Five Ways came from Cornelius Van Til (1895 to 1987), a Dutch-born theologian who taught apologetics at Westminster Theological Seminary in Philadelphia. In books such as The Defense of the Faith (1955), Van Til argued that arguments like Aquinas's begin from the reasoning of the unbeliever as if it were neutral and able to judge God, and then try to reach God by steps that the unbeliever could accept. On Van Til's view no fact is neutral, every fact is what God makes it, and the unbeliever's reasoning is itself in rebellion against God; so the Christian should argue that only the God of Scripture makes reasoning and the uniformity of nature possible at all, an approach called "presuppositional." Fesko reports that Van Til called the mixing of autonomous reason with revelation, which he found in Aquinas, "synthetic thinking," and judged that "no form of natural theology has ever spoken properly of the God who is there."²⁹³⁰ Van Til's students, including Greg Bahnsen and John Frame, carried the approach further; Frame, according to Fesko, noticed "deep tensions" in Van Til's own thought about natural knowledge.²⁹

Classical apologetics: Sproul, Gerstner, Lindsley and Geisler

Other Reformed and evangelical writers defended the use of arguments like the Five Ways. R. C. Sproul, John Gerstner and Arthur Lindsley, in Classical Apologetics: A Rational Defense of the Christian Faith and a Critique of Presuppositional Apologetics (Zondervan, 1984), argued that natural theology was the historic Reformed position, defended a cosmological argument in the tradition of Aquinas, and replied to Van Til at length.³¹ Norman Geisler, an evangelical philosopher who taught at Dallas Theological Seminary and later founded Southern Evangelical Seminary, wrote Thomas Aquinas: An Evangelical Appraisal (Baker, 1991), in which he defended Aquinas's arguments for God's existence and urged evangelicals to read him, while also setting out the points at which he, as an evangelical, disagreed with Aquinas's theology.³² Lesson 07 takes up those disagreements.

Francis Schaeffer's critique of Aquinas's view of nature and grace, in Escape from Reason (1968), is a different argument, about what happened to Western thought after Aquinas gave nature a measure of independence. It is discussed in the Liberty and the Republic lesson Francis Schaeffer, the Man and L'Abri and in lesson 07 of this module.

Claims checked

"Aquinas's Five Ways prove that the universe had a beginning."

Refuted. Aquinas wrote that the world's beginning is held "by faith alone" and "by no demonstration can it be proved" (ST I, q. 46, a. 2), and in the Summa contra Gentiles he said the "most efficacious way to prove that God exists is on the supposition that the world is eternal" (I.13).¹⁸⁶ He distinguished the per se series used in the Five Ways from an accidental series in time, and allowed that reason cannot rule out an infinite series of the second kind (q. 46, a. 2, ad 7). The argument from a beginning in time (the kalam argument) is a different argument, defended by others.

"The Five Ways say that everything has a cause, so they cannot explain what caused God."

Refuted as a reading of the text. None of the Five Ways uses the premise "everything has a cause." The premises are that whatever is moved is moved by another, that in the world there is an order of caused causes, and that things which come and go, and necessary things whose necessity is "caused by another," depend on something else.¹ Each conclusion is to something outside the class described: an unmoved mover, an uncaused cause, a being with "of itself its own necessity." The objection, popularized by Bertrand Russell in 1927, applies to an argument Aquinas did not make.¹⁹ Whether an uncaused cause is possible is a separate question.

"Aquinas rejected Anselm's ontological argument."

Supported, with qualifications. ST I, q. 2, a. 1, objection 2 states the argument in Anselm's terms ("that thing than which nothing greater can be conceived"), and the reply rejects it: it shows only that the thing "exists mentally," unless one already admits "that there actually exists something than which nothing greater can be thought."¹ He makes the same reply in Summa contra Gentiles I.11.⁶ But Aquinas does not name Anselm in either place, and he agrees that "God exists" is self-evident in itself; he denies only that it is self-evident to us. The name "ontological argument" is Kant's, from 1781.²²

"Aquinas invented the Five Ways."

Unsupported. Aquinas himself attributes the arguments from motion, from efficient causes and from grades of being to Aristotle, and the argument from governance to John of Damascus, with a hint in Averroes (Summa contra Gentiles I.13).⁶ The staff-and-hand example and the fire example come straight from Aristotle's Physics VIII.5 and Metaphysics II.1.¹¹¹³ Scholars find the Third Way's ancestry in Avicenna and Maimonides. What is Aquinas's own is the selection, the order and the compression into a single article, and the use he makes of the conclusions in the questions that follow.

"Aquinas taught that a person must prove God's existence before believing in God."

Refuted. ST I, q. 2, a. 2, ad 1 says "there is nothing to prevent a man, who cannot grasp a proof, accepting, as a matter of faith" what can be demonstrated, and ST II-II, q. 2, a. 4 gives three reasons why it is necessary for people to accept by faith even what reason can prove: it takes too long, too few have the ability or leisure, and reason errs.¹⁸

"The Fifth Way is the same as Paley's watchmaker argument, and Darwin refuted it."

Partly supported. Both are arguments from order to an intelligence, and the Fifth Way is often classed with design arguments. But Paley's Natural Theology (1802) argued from the complex fitting of parts in living things, which Darwin's theory of natural selection (1859) offered to explain; Aquinas argued from the regular tendency of all natural things, even non-living ones, to act "always, or nearly always, in the same way, so as to obtain the best result."¹¹⁶ Whether the Fifth Way is touched by Darwin depends on whether that regular tendency itself needs explaining, which is disputed among philosophers.

"Hume and Kant refuted the Five Ways."

Unresolved. Hume's Dialogues (1779) and Kant's Critique of Pure Reason (1781) contain the objections most often brought against arguments of this kind, quoted above.²¹²² Neither man discussed ST I, q. 2 directly, and modern Thomists such as Feser, Gaven Kerr and Davies argue that the objections miss Aquinas's distinctions (the per se series, the identity of essence and existence in God).¹⁵²³¹⁷ Philosophers continue to disagree; the matter is a live philosophical debate, not a settled historical fact.

"The Catholic Church requires Catholics to accept the Five Ways."

Partly supported. The First Vatican Council (1870) defined that God "can be known with certainty from the consideration of created things, by the natural power of human reason," and the Oath against Modernism (1910) added "and that, therefore, his existence can also be demonstrated."²⁵²⁶ Neither text names the Five Ways or requires any particular argument.

"Protestants reject proofs of God's existence."

Partly supported. Some do: Cornelius Van Til's presuppositional apologetics rejected arguments like Aquinas's as resting on autonomous human reason.³⁰²⁹ Many do not: Calvin wrote that God so shows himself in the universe "that we cannot open our eyes without being compelled to behold him," though he judged the knowledge from creation insufficient without Scripture; the Reformed scholastics used Aquinas's proofs; and Sproul, Gerstner and Lindsley (1984) and Geisler (1991) defended them.²⁸³¹³²

"The First Way depends on Aristotle's physics, which Newton disproved."

Unresolved. The examples (fire heating wood, the movement of the heavens in the Summa contra Gentiles) come from Aristotle's physics, and critics such as Kenny (1969) argued that the argument cannot be detached from it.¹⁴ Thomists reply that "motion" in the argument means any passage from potency to act, which modern physics does not deny.¹⁵ The dispute is philosophical.

Hard questions and answers

A Lutheran asks: if God's existence can be proved, why does the Creed say "I believe in one God"?

Aquinas raised this himself as the first objection of article 2. His answer is that God's existence is not an "article of faith" but a "preamble" to the articles: something faith assumes and reason can reach. But he immediately adds that someone who cannot follow a proof may hold it by faith, and in II-II, q. 2, a. 4 he argues that it is fitting for nearly everyone to do so.¹⁸ The articles of faith proper, such as the Trinity and the Incarnation, cannot be proved by reason at all (q. 32, a. 1; q. 46, a. 2). A Lutheran reader may judge that Aquinas gives reason more room than Luther did; that is a fair disagreement, and lesson 07 sets out Luther's own words on Aristotle and the schoolmen.

An evangelical asks: is the "first mover" really the God of the Bible?

Not by the end of article 3, and Aquinas does not say it is. Each way ends with a phrase about what people call "God," and question 3 begins by saying that "we cannot know what God is, but rather what He is not."²⁰ The case that the first cause is one, simple, infinite, good, knowing, willing and provident runs through ST I, qq. 3 to 26, and the case that God is Father, Son and Holy Spirit rests on revelation alone. The Summa's own "On the contrary" for article 3 is Exodus 3:14, God naming Himself to Moses; Aquinas set the Five Ways under that verse, not in place of it.

A skeptic asks: does Aquinas take the problem of evil seriously?

He makes it the first objection to God's existence in the whole Summa, and he states it in a strong form: if God were infinite goodness, "there would be no evil discoverable; but there is evil in the world." His reply in this article is brief, a sentence from Augustine's Enchiridion on God's power to bring good out of evil.¹ He gives a fuller account in ST I, qq. 48 and 49 and in his treatment of providence. A reader may find the short reply in q. 2 unsatisfying; it is meant to be answered more fully later.

A Dominican student asks: did Aquinas think the Five Ways were certain proofs or only probable arguments?

The text says "the existence of God can be proved in five ways" (probari potest), and article 2 calls the procedure a "demonstration" producing scientific knowledge.¹² In the Summa contra Gentiles Aquinas distinguishes "demonstrative and probable arguments" and places the existence of God among the truths reason can demonstrate.⁶ So Aquinas claimed certainty for the conclusion that God exists. Modern Thomists differ over whether each of the five, as written in its single compressed paragraph, is a complete demonstration, or a summary of longer arguments found elsewhere in his works.²⁴

An Orthodox reader asks: why does a Latin theologian begin with John of Damascus?

Because John's On the Orthodox Faith was, for Aquinas, a standard authority, read in a Latin translation made in the twelfth century. He appears in the first objection of article 1 ("the knowledge of God is naturally implanted in all") and in the second objection of article 2 (that only what God's essence is not can be known), and in the Summa contra Gentiles Aquinas credits him with the argument from the government of the world.¹⁶ Aquinas's reading of him, that natural knowledge of God is real but "general and confused," would be familiar to Orthodox readers who hold, with the Greek Fathers, that God's essence is beyond knowing.

Why should a reader with no interest in proofs read question 2 at all?

Because it is the doorway to the rest of the Summa: the distinction between what is known in itself and what is known to us, the method of reasoning from effects to causes, the vocabulary of act and potency, and the idea of a cause on which things depend at every moment all return in the treatises on law (lesson 05), on happiness (lesson 06) and on grace (lesson 07). And because the questions it raises, why there is something rather than nothing and whether order needs a mind, are still asked by believers and unbelievers alike.

Where to go next

Lesson 05 reads the Treatise on Law, where the eternal law, God's plan for the universe, plays the part that the first cause plays here. The review lesson, 14, gives a reading plan for the Summa that begins with ST I, q. 2. For natural law in outline from Cicero to Aquinas, see the Liberty and the Republic lesson Natural Law from Cicero to Aquinas. For the place of Aquinas in the medieval church, see lesson 01 of this module and Church History, Part A, lesson 09.

Learn more

• Summa Theologiae I, q. 2 (New Advent): The three articles in the 1920 translation, with links to every cited question.
• Summa Theologiae I, q. 2 in Latin (Corpus Thomisticum): The Leonine Latin, for checking the closing phrases of each way.
• Summa Theologiae I, q. 46 (New Advent): Aquinas on the eternity of the world, with the reply on per se and accidental series.
• Summa contra Gentiles, Book One, chapters 10 to 13 (isidore.co): The longer version of the arguments, where Aquinas names his sources.
• Anselm, Proslogion (Internet Medieval Sourcebook): The argument Aquinas answers in article 1, in Anselm's own prayerful setting.
• Hume, Dialogues Concerning Natural Religion (Project Gutenberg): Parts V and IX contain the objections quoted in the lesson.
• Calvin, Institutes, Book I, chapters 3 to 6 (CCEL): Calvin on the sense of deity and the knowledge of God from creation and Scripture.

Still being checked (7 items)

These points could not be confirmed against a primary source when this lesson was written and fact checked in October 2026. The text above states each one as uncertain.

• Positions attributed to Anthony Kenny (1969), Edward Feser (2009, 2017), Gaven Kerr (2015), Brian Davies (1992, 2014), Fergus Kerr (2002), Cornelius Van Til (1955), Sproul, Gerstner and Lindsley (1984) and Norman Geisler (1991) are summaries; none of these books was opened for this lesson. Van Til's quoted words come only through J. V. Fesko's 2023 article.
• That the Third Way draws on Avicenna and Maimonides is reported as the view of modern historians without a specific citation.
• William Paley's Natural Theology (1802) and its watch analogy are summarized, not quoted from an opened copy.
• The date of Anselm's Proslogion (about 1077 to 1078) is the commonly given date and was not checked in a source for this lesson.
• The Latin translation of John of Damascus used in the West is described as twelfth century without a citation.
• The life dates given for Aristotle, Avicenna, Maimonides, Hume, Kant and Van Til are standard reference dates and are not separately footnoted.
• The Sourcebook's modernized Deane text of Proslogion ch. 2 also prints "there is doubt that there exists" where Deane's sense requires "no doubt"; that sentence is not quoted.

Notes

1. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, First Part, Question 2, "The existence of God," articles 1 to 3 (ST I, q. 2, aa. 1 to 3) (retrieved Nov. 5, 2026). Long dashes in the printed text are shown as commas or colons. Read it online. ↩
2. Thomas Aquinas, Summa Theologiae I, q. 2 (Latin, Leonine text), Corpus Thomisticum, ed. Enrique Alarcon, University of Navarre (retrieved Nov. 5, 2026). Read it online. ↩
3. Thomas Aquinas, Summa Theologiae, English Dominican translation (1920), First Part, Question 2, prologue, Christian Classics Ethereal Library (retrieved Nov. 5, 2026). Read it online. ↩
4. Anselm of Canterbury, Proslogion, chapters 1 and 2, translated by Sidney Norton Deane, St. Anselm: Proslogium; Monologium; An Appendix in Behalf of the Fool by Gaunilo; and Cur Deus Homo (Chicago: Open Court, 1903; reprinted 1926), as modernized in the Internet Medieval Sourcebook, Fordham University (retrieved Nov. 5, 2026). Read it online. ↩
5. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, First Part, Question 3, article 4, "Whether essence and existence are the same in God?" (ST I, q. 3, a. 4) (retrieved Nov. 5, 2026). Read it online. ↩
6. Thomas Aquinas, Summa contra Gentiles, Book One: God, translated by Anton C. Pegis (Garden City: Hanover House, 1955), chapters 2, 10 to 11 and 13 (I.13.2 to 3, 29 to 30, 33 to 35), as given at isidore.co; quoted briefly (this translation is not public domain) (retrieved Nov. 5, 2026). Read it online. ↩
7. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, First Part, Question 12, article 12, "Whether God can be known in this life by natural reason?" (ST I, q. 12, a. 12) (retrieved Nov. 5, 2026). Read it online. ↩
8. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, Second Part of the Second Part, Question 2, article 4, "Whether it is necessary to believe those things which can be proved by natural reason?" (ST II-II, q. 2, a. 4) (retrieved Nov. 5, 2026). Read it online. ↩
9. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, First Part, Question 1, article 8, reply to objection 2 (ST I, q. 1, a. 8, ad 2) (retrieved Nov. 5, 2026). Read it online. ↩
10. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, First Part, Question 13, article 11, "Whether this name, HE WHO IS, is the most proper name of God?" (ST I, q. 13, a. 11) (retrieved Nov. 5, 2026). Read it online. ↩
11. Aristotle, Physics, book VIII, chapter 5 (256a), translated by R. P. Hardie and R. K. Gaye (Oxford), Internet Classics Archive, MIT (retrieved Nov. 5, 2026). Read it online. ↩
12. Aristotle, Metaphysics, book XII (Lambda), chapter 7 (1072a), translated by W. D. Ross (Oxford, 1908), Internet Classics Archive, MIT (retrieved Nov. 5, 2026). Read it online. ↩
13. Aristotle, Metaphysics, book II (Alpha the Lesser), chapters 1 and 2 (993b to 994a), translated by W. D. Ross (Oxford, 1908), Internet Classics Archive, MIT (retrieved Nov. 5, 2026). Read it online. ↩
14. Anthony Kenny, The Five Ways: St. Thomas Aquinas' Proofs of God's Existence (London: Routledge and Kegan Paul, 1969). Book not opened for this lesson; his positions are summarized from standard accounts and should be checked against the book. ↩
15. Edward Feser, Aquinas: A Beginner's Guide (Oxford: Oneworld, 2009), chapter 3, and Five Proofs of the Existence of God (San Francisco: Ignatius Press, 2017). Positions summarized, not quoted; the books were not opened for this lesson. ↩
16. William Paley, Natural Theology; or, Evidences of the Existence and Attributes of the Deity (London, 1802), chapter 1 (the watch). Summarized, not quoted. Read it online. ↩
17. Brian Davies, The Thought of Thomas Aquinas (Oxford: Clarendon Press, 1992), and Thomas Aquinas's Summa Theologiae: A Guide and Commentary (New York: Oxford University Press, 2014). Positions summarized, not quoted. ↩
18. Thomas Aquinas, Summa Theologiae, literally translated by Fathers of the English Dominican Province, second and revised edition (London: Burns Oates and Washbourne, 1920), online edition by Kevin Knight, New Advent, First Part, Question 46, article 2, "Whether it is an article of faith that the world began?", body and reply to objection 7 (ST I, q. 46, a. 2 and ad 7) (retrieved Nov. 5, 2026). Read it online. ↩
19. Bertrand Russell, "Why I Am Not a Christian," lecture delivered Sunday, March 6, 1927, at Battersea Town Hall under the auspices of the South London Branch of the National Secular Society; printed as a pamphlet (London: Watts, 1927). Text as given at Drew University (users.drew.edu), with the headnote giving the date and venue; Russell there recalls reading at eighteen, in John Stuart Mill's Autobiography, the sentence about the question "Who made God?" Summarized and quoted briefly (retrieved Nov. 5, 2026). Read it online. ↩
20. Thomas Aquinas, Summa Theologiae, English Dominican translation (1920), First Part, Question 3, prologue, Christian Classics Ethereal Library (retrieved Nov. 5, 2026). Read it online. ↩
21. David Hume, Dialogues Concerning Natural Religion (London, 1779), Part V and Part IX, Project Gutenberg eBook 4583 (retrieved Nov. 5, 2026). Read it online. ↩
22. Immanuel Kant, The Critique of Pure Reason (first edition 1781, second edition 1787), translated by J. M. D. Meiklejohn, Transcendental Dialectic, book II, chapter III ("The Ideal of Pure Reason"), sections 4 to 6, and preface to the second edition, Project Gutenberg eBook 4280 (retrieved Nov. 5, 2026). Read it online. ↩
23. Gaven Kerr, Aquinas's Way to God: The Proof in De Ente et Essentia (New York: Oxford University Press, 2015). Summarized, not quoted; the book was not opened for this lesson. ↩
24. Fergus Kerr, After Aquinas: Versions of Thomism (Oxford: Blackwell, 2002). Summarized, not quoted; the book was not opened for this lesson. ↩
25. First Vatican Council, Session 3 (April 24, 1870), Dogmatic Constitution on the Catholic Faith (Dei Filius), chapter 2 and canons on revelation, canon 1, English translation as given at Papal Encyclicals Online (retrieved Nov. 5, 2026). Read it online. ↩
26. Pius X, The Oath against Modernism (1910), English translation at Papal Encyclicals Online (retrieved Nov. 5, 2026). On its issue in the motu proprio Sacrorum antistitum (Sept. 1, 1910) and its rescinding by the Congregation for the Doctrine of the Faith in 1967, see New Catholic Encyclopedia, "Modernism, Oath Against," at encyclopedia.com (retrieved Nov. 5, 2026). Read it online. ↩
27. John Calvin, Institutes of the Christian Religion, translated by Henry Beveridge (Edinburgh: Calvin Translation Society, 1845), book I, chapter 3, section 1, Christian Classics Ethereal Library (retrieved Nov. 5, 2026). Read it online. ↩
28. John Calvin, Institutes of the Christian Religion, translated by Henry Beveridge (1845), book I, chapter 5, sections 1 and 14, Christian Classics Ethereal Library (retrieved Nov. 5, 2026). Read it online. ↩
29. J. V. Fesko, "Natural Theology and Cornelius Van Til," Credo Magazine, August 7, 2023, quoting Van Til (including a letter to Francis Schaeffer) and John Frame (retrieved Nov. 5, 2026). See also Fesko, Reforming Apologetics: Retrieving the Classic Reformed Approach to Defending the Faith (Grand Rapids: Baker Academic, 2019). Read it online. ↩
30. Cornelius Van Til, The Defense of the Faith (Philadelphia: Presbyterian and Reformed, 1955). Position summarized, not quoted; the book was not opened for this lesson. ↩
31. R. C. Sproul, John Gerstner and Arthur Lindsley, Classical Apologetics: A Rational Defense of the Christian Faith and a Critique of Presuppositional Apologetics (Grand Rapids: Zondervan, 1984). Summarized, not quoted. ↩
32. Norman L. Geisler, Thomas Aquinas: An Evangelical Appraisal (Grand Rapids: Baker Book House, 1991). Summarized, not quoted. ↩