1. Having proved the demonstrability of God, St. Thomas proceeds to offer a series of demonstrations of God’s existence. Since this article is both the most widely read and the most frequently misread in St. Thomas’s corpus, it merits a lengthy discussion.
2. St. Thomas opens the article with two objections to the existence of God. The first is from the existence of evil. The second is from the sufficiency of other causes to explain the existence and nature of the world. These remain the simplest and most widely used arguments against belief in God’s existence today. We will treat them more extensively when we come to St. Thomas’s replies.
3. For his Sed Contra, St. Thomas uses the words of God as reported in the book of Exodus: “I am Who am.” If an argument from authority is to be made for God’s existence, there is no higher authority to appeal to in the question than that of God himself declaring his own existence.
4. In the corpus of the article, St. Thomas famously offers five ways (quinque viae) of demonstrating the existence of God: from motion, from efficient causation, from possibility and necessity, from gradations of perfection, and from the governance of the world.
5. A note on the background of the five ways. They are not absolute proofs of the existence of God. It is possible to reject them. St. Thomas is aware of this, and we will point out ways of rejecting each of the five ways as we present them. The fact that the five ways are not absolutely binding on the mind of anyone to whom they are presented is not a deficiency, but a necessary fact, given the form of demonstration employed and the character of the human intellect. Arguments which pretend to bind the intellect absolutely are generally either metaphysically deficient or mistaken about the character of the knowledge they are trying to achieve.
6. The first way has the short form:
A. Some things are in motion.
B. Everything moved is moved by another.
C. There cannot be an infinite regress of movers.
D. Therefore there must be something which, though unmoving, is the source of all other motion.
E. We give the name “God” to a being which, though utterly unmoving, is the source of the motion of all other things.
F. Therefore God exists.
6. In giving a fuller form of the argument, we should begin with some definitions. Motion here means “any reduction from potency to act”, where act is the current actuality of a thing (what it is), and potency is the potential actuality of a thing (what it could become).
7. Some things are in motion. This premise is rejected by both the Eleatics and (in a different way) by Heraclitus. The Eleatics deny both the distinction of things and the reality of change, since being is radically one, true, and eternal. Heraclitus denies the reality of change in a different way, by asserting that change is the constitutive feature of being, so that from moment to moment there is no “one”. Since the notion of change presupposes the continuity of a substrate the characteristics of which are modified, Heraclitus (interpreted this way) denies the reality of change. Both of these positions are somewhat uncommon and come at great cost, philosophically, as they require an elaborate logical framework in order to credibly explain ordinary experience.
8. Things cause motion only insofar as they are in act. To accept this premise is to accept that only existing things (i.e. things in act) can bring other things into being. St. Thomas is positing a universe of beings, in which the ultimate explanatory principle of any event is not a rule but a thing. A full reflection on the ontology implicit in this claim would bring one into contact with modern critics of metaphysics (Nietzsche, Heidegger, Comte) and give a great deal of depth to one’s understanding of Thomistic theology. One way of rejecting it is to reject the “Principle of Sufficient Reason”, and posit the existence of brute facts (facts which, though not necessary in themselves, are not made true by things beyond themselves.)
9. Things can be moved only insofar as they are in potency to that toward which they are being moved. This premise follows from the definition of motion given above.
10. Nothing can be both in potency and in act in the same way at the same time. This follows from the definitions of potency and act, and from the law of non-contradiction. One can reject this premise by rejecting the concept of potency (as do the Eleatics) or by rejecting the principle of non-contradiction (as do some Heracliteans).
11. Therefore (from 8,9,10), whatever is moved is moved by something other than itself. Everything moved is moved by another. (Omne quod movetur ab alio movetur.)
12. If that which is causing motion is being moved, it must in turn be moved by another. Notice that St. Thomas speaks of a simultaneous series of movers, and not a temporal sequence of movers. The idea is not a line of dominos, each of which fell because some other previously knocked it over. Instead, we might think of a chain, each link of which has motive force because it is receiving that motive force from a previous link.
13. But this series cannot proceed to infinity, since without a first mover there cannot be any subsequent motion. This can be seen clearly if we consider the set of moved movers as a composite object. Since we are considering simultaneous movement and not a successive series of movements, all the movers exist at the same time and can thus be considered together. But by our earlier premise (8), this is impossible, since it would leave the entire set without a mover. Though in an infinite regress we cannot point to a part of the series which lacks a mover, we can point to the series entire, which very much lacks a mover.
14. Therefore there must be a first mover, which is in no way moved.
15. To this sort of thing is customarily given the name “God”.
16. Therefore God exists.
[To be continued in subsequent posts.]