Punkforchrist and Tanya will participate in a formal discussion on the following topic:
The Thomistic Cosmological Argument
Punkforchrist (going first) will take the theistic position and Tanya will take the atheistic position. The discussion will have five rounds, per the parameters agreed prior.
All members can comment on this formal discussion (except for the participants) in the Peanut Gallery set up in the Religion forum.
Enjoy the discussion!
I will defend this argument in two different ways: 1) using a modest version of the Principle of Sufficient Reason (PSR); and 2) appealing to a weak version (W-PSR) that is sufficient even if one rejects the traditional version.
Some Preliminary Definitions
We observe that there are things in a state of flux – that is, changing things exist. I will assume that we are in agreement on this, unless of course, you take a strong monist interpretation of reality. We can revisit this point as we continue if there is any disagreement, or need for clarification.
Now, analytically-speaking, it is manifest that everything that changes is composed of actuality and potentiality. These technical terms need not be intimidating. Actuality is simply what something is; potentiality is what it could be. So, for instance, an acorn is merely an acorn in actuality, whereas it is something else in potentiality, such as an oak tree.
These changing things are said to be in motion. The explanation of these moving entities I maintain is what Aristotle referred to as the Unmoved Mover, which is purely actual (Pure Act), since it is itself not in motion. This is the first step of the argument. The second is a demonstration that the Unmoved Mover possesses attributes most consonant with classical theism: immutability, eternality, unity, omnipotence and omniscience.
Argument Based Upon the PSR
Imagine you and I are walking to a coffee shop. On our way we discover an elephant standing in the middle of the road. You ask me, “how did that elephant get there?” and I reply, “it's just there; there is no explanation.” You would surely look at me like I'm either crazy, or merely jesting. For, it is a natural consequence of rational inquiry that we look for explanations of things. This is true of relatively small things (elephants, people, coffee shops), and it is also true of large things (continents, planets, galaxies, supernovae). We can put the first premise of the argument like this:
1. Everything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause.
A strong version of the PSR (S-PSR) would state that every fact or state of affairs has an explanation. The PSR, as stated above, makes the more unassuming claim that every existing thing has an explanation of its existence.
We can now apply the PSR to the existence of motion. Why is anything in motion at all? It is plausible, given our first premise, that:
2. If things in motion have an explanation for their existence, that explanation is an Unmoved Mover.
(2) states that if motion has an explanation at all, then the explanation must transcend the set of moving entities, and therefore is an Unmoved Mover. For, if the explanation were itself in motion, then it would be part of the set of moving entities, which entails that the explanation is not external to the set.
Perhaps you believe that the things in motion exist by a necessity of their own nature, which would satisfy (1). We can look at two distinct arguments against this.
First, our modal intuitions clearly support the notion that things in motion are radically contingent. Imagine an alternate universe that looks exactly the same as our own. Plants still grow from seeds, the Red Sox still won the 2007 World Series, and Democrats continue to raise taxes (just kidding!). Even though everything looks the same, the fundamental particles that compose each moving entity are all different from the ones that exist in our own universe. The implication, then, is that our dynamic universe is not something that exists by some logical necessity. As a result, we can safely conclude that the set of moving things requires an external cause, an Unmoved Mover.
Secondly, there is the problem of circularity in appealing to the necessity of some thing, or things, in motion. Take the example of acorns and oak trees. For the sake of argument, we may permit an infinite regress, but such a regress still requires an external cause. If every oak tree is explained by an acorn, and every acorn is explained by an oak tree, then without an external cause what we have is this: the activity of the members of the set of oak trees is explained by the activity of the members of the set of acorns, and the activity of the members of the set of acorns is explained by the activity of the members of the set of oak trees.
This explanation is circular and clearly fails to answer the question of why the set of acorns and oak trees exists at all. 
3. There exist things in motion.
This premise is apparent to our senses. As a result, it follows that:
4. Therefore, motion is explained by an Unmoved Mover.
Argument Based Upon the W-PSR
Perhaps you are not entirely convinced by the above argument. You may be inclined to accept a weaker version of the PSR. We can state it like this: everything that exists possibly has an explanation of its existence. Working with this, we can put the first premise of the so-called Modal Cosmological Argument (MCA) like this:
1. There possibly exists an Unmoved Mover.
This statement is even more modest than what we looked at in the first argument. In order to reject (1), a contradiction in the idea of an Unmoved Mover must be brought to surface. For, if an Unmoved Mover exists in just one possible world, then (1) is correct.
Now, what can we possibly infer from (1)? As a matter of possible worlds semantics, we know that:
2. Whatever is possible is either contingent or necessary.
Something is said to be contingent if it exists in one, but not all, possible worlds. On the other hand, a thing is necessary if it exists in all possible worlds. As you can see, this premise is true by definition. We can now move onto the next premise:
3. Whatever is contingent can be actualized.
If we continue with the example of acorns and oak trees, we can see that both exist contingently. Yet, it is also true that if one exists, then the other can possibly be actualized. Notice this isn't the same as saying that it is actualized; it need only be actualized in some possible world. This is a major problem for the person who accepts (1), but rejects the notion of an Unmoved Mover, since:
4. An Unmoved Mover cannot be actualized.
Actualization requires change; but an Unmoved Mover is, by definition, changeless. From this we can soundly infer that:
5. Therefore, an Unmoved Mover exists necessarily.
The W-PSR proof can be demonstrated with the use of modal logic, as well. Let x = entity; y = Unmoved Mover; and z = actualized.
1. ◊ y (x)
2. ◊ (x) → (◊ x & ~ □ x) ^ (□ x)
3. ◊ z (◊ x & ~ □ x)
4. ~ ◊ y
5. .: □ y
The Gap Problem
It's reasonable to ask: why does the Unmoved Mover have to be God? Why can't it just be some impersonal entity? The second part of any cosmological argument is to close the gap between First Cause/Necessary Being/Unmoved Mover and God.
We have already seen that something can only be composed of potentiality if it is subject to change. Now, the Unmoved Mover is changeless, which means it cannot be composed of any potentiality. Therefore, the Unmoved Mover is pure actuality (Pure Act). I believe we can infer a number of attributes about Pure Act, besides immutability: eternality, unity, omnipotence and omniscience.
The Unmoved Mover must be eternal, since to come into or go out of existence requires the potentiality to do so. Given that the Unmoved Mover is not composed of potentiality, it must exist at all times, and is therefore eternal.
Next, the Unmoved Mover must be one. For, if there were more than one Unmoved Mover, then there would be distinctions between them. But, distinctions entail limitations, and limitations entail potentiality. Because the Unmoved Mover is Pure Act, it must be one.
Finally, the Unmoved Mover must be omnipotent and omniscient. Partly actual beings, like ourselves, possess some power and some knowledge. We can therefore infer that something purely actual would possess all power and all knowledge. Based on these reflections, we may conclude that a single changeless, eternal, omnipotent and omniscient being exists. This matches the classical theist's definition of God, which means, via the transitive axiom, that God exists.
This isn't a debate per se, so if you'd like, we can also explore some other reasons for thinking that Pure Act exists. In addition to the two arguments above, I'm also thinking of a variation of Thomas' so-called “Third Way”. I look forward to your thoughts.
 Alexander R. Pruss, The Principle of Sufficient Reason, Cambridge University Press, 2006, pp. 43-44.
The word "contingent", I use here to mean "dependent on external elements".
"Prime Mover" is the same as the "Unmoved Mover".
As I respond to your premise of:
1." Everything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause."
I can accept that all the things that physically exist are, in fact, contingent beings. My cat, for example, does not just exist. She is a creature dependent on other things, such as food, water, and air. The food chain, indeed, shows that animals are all dependent on one another, as well as plants, which are in turn dependent on air, water and sunlight. All physical beings, can thus be called contingent and have a cause.
With regards to:
2."If things in motion have an explanation for their existence, that explanation is an Unmoved Mover."
I would say that this is contradictory to the premise (1), for surely if all things require a mover, then this should apply to the Prime Mover as well? Infinite regress may, or may not, require an external cause. This is not falsifiable, as we have no way to prove that there was a cause for infinite regress. Furthermore, even if there is an Unmoved Mover, there is no proof that it was the God of Abraham. It could as well have been Odin, Vili and Ve--or even Lucifer.
I accept premise (3), however, I fail to understand the jump in logic to the conclusion of (4), as we have no way of actually verifying this, particularly with the implication that this contains a proof for any specific deity.
Hume's objection--upon which I base my own objection--is based on observable things, such as numbers. You can write what you think is the largest number in existence, and I can then add 1 to it. Equally, I can write what I think is the smallest number in existence, and you can subtract 1 from it. The sequence of those events, dare I say it, is eternal. This quote sums up rather nicely the objection to even suggesting a Prime Mover:
"In such a chain too, or succession of objects, each part is caused by that which preceded it, and causes which succeed 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 counties 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."
As I have said, the First Cause, even if I should accept your argument, need not be God, but may perhaps be the universe itself. If you say "God is necessary", then it is possible that "the universe is necessary".
Response to the W-PSR:
1.There possibly exists an Unmoved Mover.
For the sake of discussion, I will accept the notion of an Unmoved Mover as possible. However, one cannot show without mere speculation that God in any way, shape or form exists in any possible worlds--let alone all of them, as we cannot even show that there are other possible worlds.
2. Whatever is possible is either contingent or necessary.
This premise is difficult at best to accept, as one cannot verify everything in all possible worlds without being omniscient. We cannot prove that cats don't exist in all possible worlds, and the same with mice.
3. Whatever is contingent can be actualized.
If you are saying that something can be made to exist in other possible worlds, then you may be correct. A daisy can exist just as well in the universe next door as it does here--but it is impossible to truly know.
4. An Unmoved Mover cannot be actualized.
No, it cannot, as it is by definition changeless. However, if something is changeless, surely it cannot change something else? For example, I am the efficient cause of this post. To write it, I would be changed because I have moved. Sense experience, the basis of every argument for God, tells me that if I cause something, I will be changed--relating to physical position, level of energy, etc. Therefore I would suggest that nothing can change something else without changing.
5.Therefore, an Unmoved Mover exists necessarily.
This conclusion contradicts the premise of the argument. No being owes existence entirely to itself, according to this argument, so the being known as "God" should be subject to this logic as well.
With regards to the "Gap Problem", how can one even be entirely sure that the Unmoved Mover is even the Christian God? It may well be one being, but it might not even be a being we have any name for, or that is indeed omnibenevolent--as Aquinas's assumption is that any God equates to the Christian one.
I bite, now move along.
You have accepted the first premise of the PSR-argument, at least for now, so we can move on.
The second premise states: “If things in motion have an explanation for their existence, that explanation is an Unmoved Mover.” With respect to this premise, you ask:
Aristotle's dictum is not that everything is moved by another, but rather everything in motion is moved by another. If there is an Unmoved Mover, then it wouldn't require a mover, since it is itself not in motion.Tanya wrote: I would say that this is contradictory to the premise (1), for surely if all things require a mover, then this should apply to the Prime Mover as well?
In order to check the soundness of this claim, we will have to examine whether my arguments against the self-sufficiency of moving entities hold up under scrutiny. I've argued that our modal intuitions strongly suggest that the set of moving things is logically contingent. I've also argued that any kind of self-sufficiency, at least among things in motion, results in circularity, and should therefore be rejected.Tanya wrote: Infinite regress may, or may not, require an external cause. This is not falsifiable, as we have no way to prove that there was a cause for infinite regress.
I agree that the existence of an Unmoved Mover doesn't ipso facto mean that Yahweh exists. Nevertheless, I think we can rightfully conclude that other gods are inconsistent with the Unmoved Mover. Odin, for example (and the other examples you offered), are changing entities, each of them presumably composed of a physical body. Given the immutability of the Unmoved Mover, it follows that the Unmoved Mover and Odin are distinct.Furthermore, even if there is an Unmoved Mover, there is no proof that it was the God of Abraham. It could as well have been Odin, Vili and Ve--or even Lucifer.
The argument is logically valid, so if the premises are correct, then the conclusion necessarily follows. We will have to take into consideration, then, each of the three premises.Tanya wrote: I accept premise (3), however, I fail to understand the jump in logic to the conclusion of (4) . . .
You have stated that you agree with both premises (1) and (3). This leaves us with premise (2).
Regarding smallest and largest numbers, I don't think this objection of Hume's is relevant to the TCA. For, as I've suggested, even if the universe is eternal, it still requires an external cause. Before I'm accused of question-begging, I refer you again to my two arguments against the self-sufficiency of the universe's motion.
The Humean citation your refer to is known as the Hume-Edwards-Campbell Principle (HEC). The HEC is dependent on an explanation's being agglomerative: one has explained the conjunction so long as one has explained the conjuncts. So, for example, imagine that you explain the presence of each member of a baseball team, each player having a different reason for joining the team. The fact that each player's presence is explained does not sufficiently explain the conjunction. For, the preceding is compatible with a decision by the team's manager to bring in all of these players. What we see, then, is that the explanation of the conjunction is only sufficient if the explanation is an external cause of the set itself.
Moreover, Hume's objection is only relevant to the S-PSR (strong version), which states that every fact or state of affairs has an explanation. The inclusion of particles of matter within a given set is a fact about these particles of matter being arranged in a certain way. Even if: a) the S-PSR is false, and b) the HEC is true, we have yet to arrive at an explanation for the existence of the particles of matter themselves. The PSR I'm defending only necessitates that every thing has an explanation.
Another objection within your citation of Hume is that the conjunction of certain things is the result of an arbitrary act of the will. I can certainly see that being the case for things that are related only incidentally – for example, the constellations of stars. However, for things that are related essentially, I think it's more reasonable to infer a real connection, independent of our cognitive creativity. After all, acorns and oak trees are capable of producing each other precisely because of an actual relation between the two kinds of things.
I have much more to say about this, but for now I'll just make one point in passing. If the First Cause is the purely actual Unmoved Mover I've argued actually exists, then such a being must be distinct from the changing, dynamic universe. The Unmoved Mover is immutable, whereas the universe is changing.Tanya wrote: If you say "God is necessary", then it is possible that "the universe is necessary".
With respect to the first premise, you stated that we cannot know that there are other possible worlds. I should clarify that I'm using “possible worlds” not in the sense of alternate universes, but rather in the sense that modal logicians are using the term. Roughly, a world W is possible so long as each member of the conjunction W allows that W itself is coherent.
Premise (2) is actually the least controversial of all four premises. If something is contingent, then it is true in at least one, but not all possible worlds. If it is necessary, it is true in all possible worlds. The only other option is that a thing is impossible, in which case it is not-possible.
You agree with (3).
Regarding (4), our inductive conclusion should be that everything that exemplifies both actuality and potentiality changes as it moves something else. This, however, is inapplicable to something immutable.
I think I've addressed your remaining objections. Of course, we can revisit these as we continue.
ETA: Tanya has chosen to withdraw from the formal discussion. Punkforchrist may post a final statement if he wishes to.
ETA2: punkforchrist has declined to post a final statement. The formal discussion is now complete.