Classical Theism

The objection: it is in principle possible that physical things or at least things other than something pure act can lack potentiality. Therefore, you cannot deduce God's existence from the existence of motion.

To illustrate what this means, take any physical object whatsoever. You can imagine a possible world in which a physical object lacks the potentiality for things like local motion or change. This would exhuast the possibility of it having potentiality, all the while not qualifying it as something which is pure act: it may still be susceptible to time, it is composite, it isn't all powerful, etc. Therefore, the argument from change is false (well, at least it isn't deductive).

I would like at the outset, to make some clarifying statements about why the argument from motion proceeds as it does. In many, if not all variations of the argument from motion, it starts with the premise "change occurs", or something synonymous. The reason for this move is soley to establish the reality of the metaphysical categories act and potency. It is not, and I repeat, it is not the premise by which the existence of God is directly derived. To put it in a crude syllogistic form:

P1 Change occurs
P2 So, actuality and potentiality exist
P3 If actuality and potentiality exist, then God exists
C Therefore, God exists.

After the establishment of change, we get act and potency. From this, the argument (I have Edward Feser's argument in mind) then applies and seeks to understand the conclusion when these metaphysical categories are applied to actualization which is of a vertical sort, rather than a horizontal one. Meaning, it is concerned with the continued actualization of a potential, rather then the actualization of a potential throughout points in time. The argument deduces God's existence from the fact that there are objects that exist which are being continually actualizaed, not the actualization of potentialities happens in a temporal manner.

To make even more preliminary statements, however, I would like to make a distinction between the ways in which a thing can lack potentiality. These two I have labeled underivative existence and derivative existence. The first we would call God, and the second would be some sort of being which lacks the capacity to change. Understanding now what the argument's objective is, take any physical object. By virtue of being a physical object, regardless of whether or not "it lacks the capacity for change", it is always physically composite. Cups, spoons, houses, and boeing 747s are the way they are because of the various arrangement of atoms involved. Atoms can be broken down into protons, neutrons, and electrons, and further those things can be logically divided by space (if it is .0000000000000000001cm long, then we can say it is composed of two parts which are .0000000000000000001/2cm in length). Strictly speaking, anything in space is divisible (composite), and everything physical is in space. Knowing this, we can then say that call physical things are dependent on their subsidiary parts for their existence. Meaning, the arrangement of parts actualize the potentiality for there to be a whole. Therefore, in order to appease the causal principle, we must then posit some being which is causally prior to this. You can run this game ad infinitum, but ultimately to satisfy the chain of causality, you must at some point come to the existnece of an "unmoved mover" or "pure actuality". Physical things may very well lack the capacity to change, but it is in the very nature of the things which deems it something of derivative existence. You must therefore appeal to that which is of underived existence.

The physical is by definition always divisible. For example, if I take a stick and cut it in half, I can then cut one of those in half, and then one of those in half, ad infinitum. Because physical things are the way they are, there can always in principle be a division between it. Therefore, every physical thing, just in terms of being physical, is composite. What this means then is that each physical whole is only in existence insofar as each one of its constituents continues to hold it in existence or cause it to be. Metaphysically, the potential for the whole is actualized by its parts. Therefore, you must appeal to something ontologically prior in order to explain its existence or we end up with a "brute fact" you might say.

A really important distinction to make when speaking of act and potency is the potential for accidental changes lets say (this involves moving up or down, left to right, etc. Any change something undergoes that is not a change in the thing itself), and the "potential for existence". There are things of derivative existence (books, particles, angels, atoms, etc), and things of underived existence (this is God). Given the analysis above, an atom is strictly speaking something of derivative existence because it must causally apeal to its structure. In order to satisfy the causal principle, you must at some point in the chain of continued actualization end up with something of underived existence. Sure, you can posit that in some way a physical thing can have no potentiality. There is nothing, as far as I can tell, logically wrong with that; however, given the reasons already stated, you will still need something causal prior to it in order to explain its existence as such.

Another classic thomistic appraoch, even though I am not a huge fan of it, would be to appeal to its metaphysical composition (technically, a composition of act and potency is a metaphysical composition, but that isn't what I am talking about). That is, to state that every physical thing is composed of matter and form (it has material existence, and has a physical/formal structure) or to state that every physical thing is composed of essence and existence (it exists, and it has a nature). In either case, you cannot appeal to one or the other to explain a physical things existence because you will end up with something circular: the form causes the matter to exist, which causes the form to exist, which causes the matter to exist, etc; its existence is caused by its essence, which is caused by its existence, which is caused by its essence, etc. You must then appeal to something which is not composed of form or matter, or of essence and existence. This is what we call, God.

It is important when talking about the argument from change to understand why change is spoken about at all. As I see it, change is simply talked about in order to establish the existence of act and potency. It is not the change itself which is important to the deduction, it is the metaphysical principles it demonstrates. When Feser (I am assuming you've read some of his work. If you haven't, you really should) talks about change, he doesn't really deduce God existence from that change, even if it seems like he is doing so. Rather, he deduces God existence using the metaphysical principles change proves, and uses them in a way to show that everything but God is only potentially in existence, whereas He is actually in existence; He is pure act.

Sorry for the long winded answer. This is something I struggled with for awhile and I feel a very thorough explanation is required. If you have any further questions or would like me to clarify something, I will be happy to respond.

What are the best arguments for accepting teleology as a real feature of the natural world?  Are there any good contemporary accounts/defenses of teleology in nature?

I am thinking mainly historically involved books, but anything would be fine.

That's more or less what I'm thinking my response would be, especially given that God is a being of pure act, His essence is His existence, etc.