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A DEFENSE OF AQUINAS’S FIRST WAY

            In this article, the fourth article in the blog series Contending for God, sponsored by the Canadian Apologetics Coalition, I will first outline Thomas Aquinas’s first way, then comment on the premises.[1] Aquinas says that this is the most evident or manifest way to demonstrate that God is.[2] It should be noted that if this argument demonstrates that a divine being exists, it is not a demonstration, or a making manifest, of the nature of God, but, of the fact that God exists, or is. It is a demonstration that only claims to show that something is, not what it is. It is also necessary to note that there are two types of demonstration. Demonstration propter quid (by way of the thing that is)[3] moves from the essence or nature of a cause to its effects.[4] Demonstration quia (demonstration of the fact)[5] moves from the effect to the existence of the cause.[6] In light of the above observations, it is not necessary to know anything about the nature of a thing prior to showing that the thing in question exists. However, in showing that a thing exists one is gaining knowledge of the thing in question. The first way is an example of demonstration quia. Finally, note that this particular demonstration, if it is valid and sound,[7] does not necessarily demonstrate the existence of the triune God of the Christian New Testament; no philosophical demonstration can move from our knowledge of the world to the existence of the triune God of the Christian New Testament. If this demonstration is successful, all that it shows is that some form of Theism is true.

            The first way can be outlined as follows:[8]

(1)   For it is certain and evident to the senses that something in this world moves.
(2)   But, all that is moved, is moved by another.
a.       Nothing is, in fact, moved, unless it is in potency[9] to that towards which it is moved.
b.      But, a thing moves only insomuch as it is in act.[10]
c.       In fact, to move is nothing else than to bring a thing from potency to act.
d.      But, it is not possible that a thing be reduced to act, unless it is by a thing that is[11] in act.
                                                              i.      For example, an actually burning or hot thing, such as a flame, makes wood, which is potentially burning, to be burning in act, and in this way the wood is moved and altered.
e.       But it is not possible that the same thing be simultaneously in act and in potency in the same sense, but only in a different sense.
                                                              i.      That, in effect, which is actually burning, cannot at the same time be potentially burning, but is simultaneously potentially freezing/cold.
f.       It is therefore impossible that, in the same way and same sense, a thing is moving and is moved,[12] or in other words, that it moves itself.
g.      It follows, therefore, that all that is moved is moved by another.
(3)   If, therefore, that which moves is moved, then it must be moved by another; and this by another [and so on].
(4)   But this cannot proceed to infinity:
a.       Because, in this case, there would be no first mover; and consequently, no thing would move another,
b.      Because second movers do not move unless they are moved by a first mover, in the same way that a cane is not moved unless it is moved by a hand.
(5)   Therefore, it is necessary to arrive at (or come to) a first mover which is not moved:
a.        and this is what all consider to be God.[13]

Aquinas’s demonstrations of the fact that God exists are frequently misunderstood. This is due partially to a language barrier, and partially to certain philosophical principles that were rejected, or improperly explained, by a number of important modern philosophers such as Descartes, Leibniz, and the like. Therefore, in order to properly understand this demonstration, one must take the time to understand the terms, and what Aquinas presupposes for this particular demonstration.[14]

Before we look at the premises, we must first consider a principle that is foundational to this argument – the principle of causality. This principle states that anything that does not have in its own being that which sufficiently explains the totality of its existence must receive its existence from another (an efficient cause) that exists by, and for, itself.[15] Briefly put, this principle maintains that nothing comes from nothing, or that “Every being that begins to exist needs a cause.”[16] To deny this principle would be to claim either that it is possible for a thing to be the cause of its own existence, or that things can come into being without cause. With this principle in mind let us take a look at the premises.

First, when Aquinas talks about movement he is talking about change, as can be seen from the definition that he gives of change (to move) as: to bring a thing from potency to act. Change is, therefore, the actualisation of a potency.[17] That something changes is one of the most obvious of all empirically observable facts.[18] Unless one is willing to accept absolute monism (or some form of Parmenides’s doctrine of Being), in which all that exists is Being, one is obligated to recognize the truth of the first premise. Even for an absolute Idealist, in which all that exists is in the mind, something changes; that is, the mind states, of whatever mind happens to contain all things, change.[19] As such, this argument begins with empirical data which is evident to all, and undeniable without an absolute rejection of one’s own being.[20] The first premise, something moves, is therefore true.

The second premise is a corollary of the principle of causation, and in light of the above explanations, needs no elaboration.[21] Note that Aquinas, following on the minimal claim of premise 1, only states that all things change. Logically there need be no more than one changing thing for premise 2 to be true, as it applies the principle of causation to changing things, regardless of whether there are many or few.

The third premise is simply a further application of the principle of causation, and an extension of the second premise. If premise 2 is true, then premise 3 follows necessarily. Premise 3 is essentially the positive form of premise 4. It seems that premise 2 pushes us, necessarily into an infinite regress of moved movers. Premise 3 is only part of an argument that Aquinas assumes is obvious (see fn. 8). Namely, if something is moved, then either there is an infinite regress of moved movers, or there is an unmoved mover which is the cause of the whole series of moved movers. Once this dilemma is assumed, Aquinas can then move onto the final argument, which is a negation of one of the possible options in the dilemma.

The fourth premise claims that an infinite regress of changed changers is impossible. This claim is based upon the reflection that there are two basic types of causal series: Accidental causal series and Substantial/essential causal series. An accidental causal series is a series (cause-effect) in which the cause is temporally prior to the effect, and in which the cause is not absolutely necessary for the continued existence of the effect.[22] An essential causal series is a series (cause-effect) in which the cause is simultaneous with the effect, and therefore, is essentially necessary for the continued existence of the effect.[23] A series of changed changers is an essential causal series. For the actualisation of any potency the actualizing agent must be continually present throughout the process of becoming.[24] To deny premise 4 would be the equivalent of saying that if a paintbrush is long enough, then it can paint the Mona Lisa by itself.[25]

In light of the above observations the conclusion follows necessarily from the premises, each of which are true. Therefore, a being exists which is the primary unmoved mover of all movement - the unchanging changer – the prime mover. This argument concludes by demonstrating the existence of an unchanging being that is the first cause of all change. If at any moment this unchanging changer ceased to cause change, then all changing things would cease to exist. If no such being exists, then nothing exists, and if something exists that changes, then this being exists. This is a minimal description of what most people would describe as a divine being.[26] Some form of Theism is, therefore, true. The next question to ask is: “Is this divine being the God of Christianity?”[27]


[1]I don’t expect, in this article, to add any new insights concerning this well-known demonstration. An innumerable amount of books and articles have already been written about this argument, both for and against it. I have not yet discovered any successful counterarguments to the first way.

[2]In the Summa Contra Gentiles, where Aquinas elaborates similar, with more in-depth treatments, arguments to demonstrate that God exists, he distinguishes between demonstrative arguments and probable arguments. Both should be used in convincing the unbeliever. Demonstrative arguments proceed from certain premises to a certain conclusion. The purpose of a probable argument is to show that the claims of Christianity which cannot be discovered by the natural intellect are not contrary to what can be discovered by the natural intellect. Cf. Thomas Aquinas, God, bk. 1, in Summa Contra Gentiles, trans. Anton C. Pegis (1975; repr., Notre Dame: University of Notre Dame Press, 2005), 77. Norman Kretzmann, The Metaphysics of Theism: Aquinas’s Natural Theology in Summa Contra Gentiles I (1997; repr., Oxford: Oxford University Press, 2004), 52, 54-55. The arguments that Aquinas presents for the existence of God would be considered demonstrative arguments.

[3]Also known as A priori Demonstration.

[4]Thomas Aquinas, Summa Theologiae I, Q. 2, A. 2. All quotations from Aquinas are my own translations.

[5]Also known as A Posteriori Demonstration.

[6]Aquinas, ST I, Q. 2, A. 2.

[7]For a logical analysis of this argument, as well as the other 4 ways, see, Paul Weingartner, God’s Existence. Can it be Proven? A Logical Commentary on the Five Ways of Thomas Aquinas (Berlin: Ontos Verlag, 2011).

[8]Aquinas, ST I, Q. 2, A. 3. This is my own rendering of Aquinas’s first way. I have arranged it thus to make salient the main premises, and the supporting premises. For example, under premises 2 and 4 Aquinas provides support for the content of these premises. Though the support does not belong to the argument as such, it is necessary for explaining what might, otherwise, be a difficult concept to understand. Other versions of the First way can be found in the Summa Contra Gentiles and the Compendium of Theology. There are other ways of outlining the first way, as can be seen from David Oderberg’s article, "’Whatever is Changing is Being Changed by Something Else’: A Reappraisal of Premise One of the First Way”, in Mind, Method and Morality: Essays in Honour of Anthony Kenny, eds., J. Cottingham and P. Hacker, 140-64 (Oxford: Oxford University Press, 2010).

The argument is logically valid. The proof is as follows:

                     A1)      M                  (Premise 1)
                     A2)      M→A            (Premise 2)
                     AC)     A                    (Modus Ponens [MP])

                     B1)      A→(IvU)       (Implied. This argument is an Enthymeme. The 3rd premise above is I.)
                     B2)      A                   (Necessary conclusion of A)
                     BC)     IvU                (MP)

                     C1)      IvU                (Necessary conclusion of B)
                     C2)      ¬ I                 (premise 4)
                     CC)     U                    (premise 5)

                Legend:
                  M = Something is moved.
                  A = moved by another.
                  I = there is an infinite regress of moved movers
                  U = There is an Unmoved mover.

                Credit for this logical proof for Aquinas's First way goes to Francisco Romero Carrasquillo who published this proof at (http://iteadthomam.blogspot.ca/2011/01/first-way-in-syllogistic-form.html). I modified it slightly, replacing first mover with unmoved mover (and, therefore, F with U).

The entire argument B is implied by Aquinas’s affirmation of I (premise 3), ¬ I (premise 4), and the conclusion U. It is not, as such, found in his explanation of the first way. It is evident from 3, 4 and 5 that Aquinas is implying that there is a logical dilemma between I and U. AC, though it is not stated in the paragraph in which Aquinas explains the first way, is the necessary conclusion of the first two premises. As a necessary conclusion of a true and valid demonstrative syllogism it is perfectly acceptable to use it, as true, in other syllogisms.

[9]The term potency is a technical term that is used to describe all of the non-actual, but possible, states of an actually existing being – a being that is in act. Other words that are sometimes used in the place of potency are potential, or possible. I prefer to use this uncommon term so as to avoid any misunderstanding that might be caused by the variation of meanings that are attached to the more common words, potential and possible. For further elaboration on the word potency, and what is implied by it, see note 17.

[10]The phrase “in act” describes the state of a thing that actually exists in the mind-exterior world. It could be paraphrased as “to be actual”, or “to be really existent at the present moment in the mind-exterior world, independent of anyone’s perceptions of it.” It is opposed to potency, which is the non-existing but potential states of an actually existing thing. That which is in potency does not actually exist, but is a possible future state of some being that is in act. For example, an apple seed is which is in act, is also in potency, an apple tree. This potency may never be actualized (or realized).

[11]I translated ens as “that is” for readability, but it should technically be translated “being” or “a being”.

[12]Is moving and is moved from act to potency. In other words, it is impossible that the same thing be, at the same time and in the same sense, in act and in potency, or actual and potential. The wood cannot be, at the same time, and in the same sense, on fire and potentially on fire. If it is potentially on fire then it is actually not on fire, and something is required to move it from being potentially on fire to being actually on fire.

[13]It should be noted that this conclusion is looking forward to that which Aquinas will then deduce from this conclusion. As Norman Kretzmann, in his commentary on the first book of Aquinas’s Summa Contra Gentiles has remarked, the attribute of a first unmoved mover, even when coupled with the conclusions of Aquinas’s other arguments, cannot be said to be God in the sense of a personal intellectual being that governs the entire universe (Kretzmann, The Metaphysics of Theism, 84-113.). So, it is a bit early to declare the existence of God, properly speaking. However, this declaration is warranted if we consider that the conclusion of this demonstration will allow us to deduce other attributes of this unmoved mover which will permit us to say, without a doubt, that we have demonstrated the existence of what can be properly called God.

[14]One thing that Aquinas presupposes for his demonstrations of the fact that God exists, as found in the Summa Theologiae, is that the reader is already familiar with the basic notions of Neo-Platonic and Aristotelian philosophy. The Summa Theologiae was, essentially, an introductory textbook to theology (“Our proposed intentions for this work are to render all that is pertinent to the Christian religion such that it is well adapted for the instruction of beginners. (ST I, Prologue)”). To study theology in a medieval university one must have already completed, at least, the trivium which included rhetoric (and study of law), grammar (and study of literature), and logic (which would have been taught using Boethius’s commentary on Porphyry’s Isagoge, which was more of a textbook in metaphysics than logic); and, most likely, the quadrivium which included studies in arithmetic, music, geometry and astronomy. Some had also completed a further year of philosophy. Therefore, the intended audience of the Summa Theologiae was people who had finished their studies in literature, grammar, languages, philosophy, and the natural sciences; beginners in theology, but people who had finished the equivalent of a modern bachelor degree in the humanities.

[15]W. Norris Clarke, The One and the Many: A Contemporary Thomistic Metaphysics (2001; repr., Notre Dame, IN: University of Notre Dame Press, 2007), 180. Clarke also notes the important difference between Aquinas’s construal of the principle of causality and that given by the empiricist tradition from Hume to recent times (Ibid., 180.), as well as Leibniz’s construal of this principle (Ibid., 181.).

[16]Ibid., 180-181.

[17]Potency could be described as the inherent principle of becoming that is within any actually existing thing. It is defined by Clarke as “any potential subject that receives and limits an act, and is the principle of continuity underlying and determining the limits of the actual changes it can go through. (Clarke, 159.)” In the glossary at the end of his book he notes that potency is “that principle within a being which limits some act or actual perfection possessed by the being and is the root of its capacity for change. (Clarke, 318.)” He also notes a distinction between passive and active potency (Clarke, 318). The potency of X is that which X is capable of becoming based upon the type of thing that X is. For example, an acorn has innumerable potencies, but to become an orange tree is not one of them. Pure potency does not actually exist, except as a being of reason. In any change there is a term from which the change occurred (which is in act), and a term to which the change occurred (which is only potentially in act based upon the nature of the term from which). Prior to the change the term to which is only possible and non-existent, while the term from which is actual. After the change the term to which is actual. Change is what happens when one of the potential terms towards which a thing is potential is brought to term. Furthermore, in deepening our understanding of change we realize that there are different types of change, such as accidental change (a change that happens to a thing, but which does not affect the nature of the thing, such as local motion) or substantial change (a change that happens to a thing such that its nature is affected, that is, it is changed from one substance into another substance, for example, when a living animal, such as a cat, dies, it goes through a substantial change).Cf. Aristotle, Metaphysics, 1065b16-17.

[18]Note that it is not necessary to claim that everything changes, or that most things change, or that you change, that I change, or that we all change. All that is claimed is that something changes. That much is evident.

[19]Change, as Thomas refers to it, is a basic empirical fact that is evident to all of humanity and integrated into all views, regardless of whether they are Buddhist or Hindu; Christian, Jew, or Muslim; Nominalist or Realist; Idealist, Rationalist or Empiricist; Existentialist or Phenomenologist; Presuppositionalist or Evidentialist;  or some strange mixture of the above.

[20]Even contemplating Descarte’s (or Augustine’s) famous formula, “I think, therefore, I am”, is to change. It is an intellectual movement from the truth of one true proposition to the necessary truth of a second proposition. Some philosophers, such as Heraclitus, even suggested that change is so fundamental to everything that all that is, is flux.

[21]Aristotle on the second premise, cf. Aristotle, Metaphysics 1049b23-26.

[22]For example, Abraham begat Isaac who begat Jacob. A white billiard ball hits the eight ball into the corner pocket.

[23]For example, a hand moving a stick moving a rock, or my whistling a tune.

[24]For example, as soon as the cause of the whistling, my will, no longer wills the whistling of the tune, the whistling of the tune stops, and the whistling of the tune continues only so long as the will that wills the whistling of the tune wills the whistling.

[25]A. D. Sertillanges, Les Sources de la Croyance en Dieu, ed. in 8vo, p. 65, quoted in Réginald Garrigou-Lagrange, God : His Existence and His Nature : A Thomistic Solution of Certain Agnostic Antinomies, 5th ed., trans. Dom Bede Rose (1934; repr., Lonely Peaks Reproductions, 2007), 207.

[26]Space does not permit, but, from the conclusion of this argument (that an unchanging changer exist) we are able to deduce that the being in question is also pure act, existence, eternal, perfect, impassible, infinite, etc. Cf. Aristotle, Metaphysics, 1072b24-29.

[27]Thanks to Shawn Ferguson who read through this publication and made many valuable suggestions.

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