Unlike the Ontological Argument, the Cosmological Argument proceeds a posteriori. It begins with a very general claim about the physical universe that is meant to be supported by observation -- e.g., the claim that some things are in motion, or that some events have causes -- and then proceeds to the conclusion that there must be a supernatural agent that somehow causes or explains this fact of experience.

Like the Ontological Argument, the Cosmological Argument comes in many versions. The most sophisticated version is due to Samuel Clarke (in A Demonstration of the Being and Attributes of God, Feinberg, p.32), and that is the version I will discuss in class. In what follows I offer an interpretation the most important historical version: Aquinas's "First Way" (Feinberg, p 30). First I'll give a sentence by sentence exegesis of the text. Then I'll try to reconstruct the argument in more explicit terms.


(Click on the picture for a link to Aquinas resources on the net)

Summa Theologica: Part I, Question 2, Article 3.

Aquinas begins with a straightforward fact of experience:

It is certain and evident to our senses that in the world some things are in motion.

It will emerge as we proceed that Aquinas uses the term "motion" in a very general sense. As we ordinarily think, motion is a special sort of change, namely change of position. But Aquinas uses the term to cover change in general. When something that is cold becomes hot, we would not ordinarily say that it has been moved. But Aquinas does say this. And this forces us to read Aquinas's talk of motion and being moved as talk of change or alteration of any sort. You should ask yourself whether this matters for the cogency of the argument.

Whether we interpret the claim in its ordinary sense on in the special sense just described, it would seem to be obviously correct. You may doubt this: looking ahead to more radical skeptical arguments, you may doubt that anything about the external world is "certain" or "evident". But for now let us set those doubts aside. So far, Aquinas speaks as the voice of common sense. And it is clear that most ordinary atheists or agnostics would happily grant him this harmless assumption.

(M) Now whatever is moved is moved by another.

This is the first crucial move; but to his credit, Aquinas does not ask us to take it on faith. Rather he gives us an elaborate subargument for it. This subargument occupies most of the rest of the paragraph. Before we get to it, however, it is important to note an ambiguity in this formulation. This sentence might mean:

Whatever is in motion is moved by another thing.

But it is might also mean:

Whenever something begins to move -- i.e., whenever something changes from being at rest to being in motion -- it is moved by another thing.

Aquinas's language certainly suggests the first reading. But as we consider the subargument, it will emerge that he probably has the second reading in mind.


Subargument for Principle M.

...for nothing can be moved except it is in potentiality to that towards which it is moved; whereas a thing moves in so far as it is in act. For motion is nothing else than the reduction 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.

This passage presents the central principle of Aquinas's physical theory, which is derived from Aristotle's account of motion and change. The central concepts of the theory are "potentiality" and "actuality", and the only explanation we get of these notions here is to be found in the example of fire and wood. Fire is actually hot; wood is only potentially hot. And the only way for something that is potentially hot to become hot is for it to be affected by something that is actually hot. Aquinas appears to take this to support the following general principle:

(P/A) If an object is potentially F, the only way for it to become actually F is for it to be affected by something that is actually F.

But you should note that Aquinas does not explicitly commit himself to this very strong claim. The principle he articulates in the third sentence of the passage might be the weaker claim:

(P/A*) If an object is potentially F, the only way for it to become actually F is for it to be affected by something that is "in a state of actuality".

This principle is weaker because it does not require that the affecting body be actually F. It might be actually G, and still manage to change a body that is potentially F into one that is actually F.

Which version of the principle does Aquinas have in mind? We face the following difficulties.


If we want to know what exactly Aquinas has in mind here, we will have to consider texts in which he discusses change and motion at greater length. If you are interested in the argument, you should certainly pursue the matter. But let's see what use Aquinas makes of these principles in the present context. That may help us to resolve at least some of our interpretative questions.

The subargument for principle M is completed as follows:

Now it is not possible that the same thing should be be at once in actuality and in 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.

This is obviously correct. To say that a thing is potentially F is to say that it is not yet F, but that it might be made to be F by a suitable cause. So clearly, a thing cannot be both potentially F and actually F. (Once you've arrived at your destination, you are no longer on your way.)

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 (M) whatever is moved is moved by another.

How are we to understand this argument?

Let's call something an F-mover just in case it makes something that is potentially F into something that is actually F, and let us say that something is F-moved just in case it undergoes a transition from being potentially F to being actually F. Now consider a situation in which an F-mover causes an F-moved to become F over a period of time t. Principle (P/A) implies that

(1) The F-mover must be actually F at the beginning of t.

But obviously enough,

(2) The F-moved. cannot be actually F at the beginning of t.

And from this we may infer:

(3) The F-moved and the F-mover are not identical.

But since the argument is fully general, not depending in any way on which property F we had in mind, we may infer the more general claim:

(4) Nothing that is not F can make itself to be F.

The most imprtant thing to note about this little argument is that it presupposes the stronger version of the principle (P/A). If we try to run the argument using the weaker, more plausible version (P/A*), we will see that it breaks down at the first step. The arm that moves the match is a "heat-mover", since it causes something that was not hot to be hot. But it is not itself hot. So we seem to have a case in which an F-mover is not actually F.


The Case for a "First Mover"

Let's set this problem aside, and continue with the argument. Aquinas proceeds as follows:

If that by which [an object] is moved be itself moved, then this also must needs be moved by another, and that by another again.

Suppose an object X is moved -- caused to change from being potentially F to being actually F -- by another object Y. Then either Y itself has been moved, or it has not. Suppose Y has been moved. Then it must have been moved by another object Z.

But this cannot go on to infinity,

This is the next crucial move. The chain of movers cannot go on to infinity. If X is moved by Y and Y by Z, we must ultimately reach some object G that is not moved by another. Aquinas calls this object, G, an unmoved mover. It is an object that has the power to bring about the transition from potentiality to actuality in other things, but which has never undergone this transition itself. It follows that G must always have been "in actuality", at least in the relevant respect. So, for example, if we are talking about ordinary motion -- change of position over time --ordinary things are moved by other ordinary things; but this chain must ultimately terminate in an object that moves other things but has never itself been moved. This is literally an unmoved mover. The argument establishes, on Aquinas's view, that such a mover must exist. He then identifies this unmoved mover with God.

But why can't the sequence go on to infinity?

because then there would be no first mover, seing that subsequent movers move only inasmuch as they are moved by the first mover; as the staff moves only because it is moved by the hand. Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God.

The main problem with this climactic passage is that the argument against the possiblity of an infinite chain of ordinary objects, each moved by some previous object, seems transparently question-begging. On the face of it, Aquinas is saying: There must be a first mover, since if there weren't there would be no first mover. But that is a terrible argument: nothing could be less convincing. Now as interpreters it is our job when confronted with what seems to be a terrible argument to root around for alternative readings. Good philosophers do sometimes make mistakes; but we should assume that they do not often make stupid mistakes. So when the most natural interpretation represents a good philosopher as making a stupid mistake, we have reason to hunt around for a better interpretation. I will leave this task to you in this case. What could Aquinas possibly be doing in the climactic passage, if he is not simply begging the question?


A Reconstruction of the Argument

Let us attempt an explicit reconstruction of Aquinas's line of thought. As before, I'll highlight the undefended assumptions in red.) We begin with a manifest fact of experience:

(1) In nature, at least one thing X that was potentially F has come to be actually F.

We now have a version of principle (M).

(2) Nothing can cause itself to become actually F.

Now (2) is supported by the Subargument, which we may represent as follows:

(a) Becoming F is a transition from being potentially F to being actually F.
(b) In general, a thing that is potentially F can come to be actually F at a certain time t only if it is affected by something that is actually F at t. (This is principle (P/A).)
(c) But nothing can be simultaneously potentially F and actually F.
(d) Therefore nothing can cause itself to become actually F.

Now we have the 'No infinite chains" principle:

(3) The chain of F-movers that terminates in X cannot be infinite.

It may be that X was first moved by Y, and Y by Z. But eventually we reach an object that is capable of moving other things but which has never been moved itself.

(4) Therefore there must be a first F-mover that has never itself been moved.

(5) Therefore, God exists.


An Evaluation of the Argument

I should stress that I am not an authority on the interpretation of Aquinas; so you should feel free to object to my reconstruction. Indeed I would be delighted if one you could provide a convincing alternative account of what Aquinas is up to, since the argument as I have reconstructed it is simply no good. I'll list only the more important problems.


(A) As we have already noted, the principle (P/A) seems to be false. An object that is not itself red can make another object red, as when an invisible flame causes an iron bar to glow. But this seriously undermines the subargument. As I have mentioned, if we retreat to the weaker principle (P/A*), the argument doesn't work. The weaker principle requires that whatever causes something to become F must be actual in some respect. But a resting animal is, for example, actually warm. So it is compatible with this principle that a resting animal should cause itself to move (which is what common sense suggests in any case); but this is incompatible with Aquinas's central claim that nothing moves itself.

(B) There is an unstated assumption in the transition from (2) to (3). We begin with an object that comes to be F, and principle (2) tells us that it cannot have caused this change in itself. Premise (3) then argues that the chain of causes that terminates in X's coming to move cannot be infinite. But here we seem to have presupposed that

(*) If an object changes in some respect, there must be some cause of this change.

We can agree that X cannot cause itself to move without conceding that something else must have caused it move. Why can't X just start moving for no reason at all? Aquinas assumes, in other words, that every motion must either be self-caused or caused by an external thing. But it seems possible to imagine motion that is not caused at all. Aqunas seems to have ruled this out withoiut argument.

(C) As noted above, the "No Infinite Chains" principle has no clear support, and it is not intuitively obvious. It may well be that Aquinas has an argument in mind here. But I cannot say what it might be.

(D) Aquinas seems to assume that the only alternatives are:


But there is another possibility:


(E) The argument appears to have some bizarre consequences. If we accept the (P/A) principle in its strong form, the argument apparently implies that God is constantly in motion. Perhaps this is not so strange. But note that the argument apparently applies to every sort of alteration in which something that is potentially F comes to be actually F. If God is to be the ultimate cause of every such alteration, then not only must God be in motion; he must be hot; he must be red , and so on. And not only this. If we take Anselm's example seriously, we must believe that only a hot thing can make a cold thing hot. But by parity of reasoning, only a cold thing can make a hot thing cold. If a single entity, God, is to serve as the ultimate cause of every "alteration chain", not only must God be hot, he must also be cold. But this is not just strange. It is absurd.

(F) It should be noted that the despite Aquinas's persuasive wording, the argument does not show that the Prime Mover is anything like God as traditionally conceived. For all we are given here, the Prime Mover might be a star or a planet in perpetual motion. Moreover, the argument does nothing to establish that there is only one first mover. Even if we grant that each chain must terminate in an unmoved mover, it may be that there are many such entities, each responsible for some but not all of the alteration we find in nature.