Over the years I've had two reservations about the argument from contingency. The first reservation comes from the possibility that there are brute facts. The argument from contingency depends on the principle of sufficient reason which states that for anything that exists, there is a sufficient reason for why exists rather than not existing. If there are brute facts, then it could be that some things exists for no reason at all. They're just there, and that's all there is to it.
I am highly skeptical of brute facts, but I haven't been able to rule out the possibility completely. But supposing there are no brute facts, that brings me to the second reservation about the argument from contingency. Is the universe a contingent thing? It's hard to say.
If there is a necessary being, and the universe is it, then there would be no need to postulate anything beyond the universe to explain the existence of the universe. But if the universe is contingent, and there are no brute facts, then there must be something beyond the universe that explains the existence of the universe.
Ultimately, if there is anything at all that's contingent, then there must be something that exists by necessity. Ultimately everything contingent that exists must be traced back to something that exists by necessity. In other words, there must be something that exists, and it's impossible for it to not exist.
While I have no problem deducing the fact that there is at least one necessary being, I have a hard time wrapping my mind around anything actually being a necessary being. Whenever I mull over whether any given object is a necessary being, I do two things. First, I try to conceive of it not existing. Second, I try to see if there is any contradiction in the supposition that it doesn't exist.
Although I can see that there must be a necessary being of some sort, whenever I try to imagine such a being, it's just as easy for me to imagine it not existing. God, of course, is the being I think is necessary. But unless I stipulate from the get-go that God is necessary, there's nothing else in the concept of God other than the stipulation that prevents me from imagining his non-existence. So apparently, conceivability cannot help me answer the question of whether or not any given entity is a necessary being or not. The mere fact that I can imagine the universe or God not existing doesn't entail that either is a contingent thing.
But neither can I deduce a contradiction in the denial of the existence of the universe or God. It seems, at least prima facie, that there is a possible world in which God exists, but the universe does not. It also seems, at least prima facie, that there is a possible world in which the universe exists, but God does not. It even seems, prima facie, that there is a possible world in which nothing at all exists. The only way I can see to deduce a contradiction from the non-existence of any named entity or being is to add propositions to it that we have reason to think are true.
We can deduce that God is a necessary being by adding the proposition that God is the source of everything else that exists. If God is the source of everything else that exists, and everything must be traced back to some necessary being, then it seems to follow that God is the necessary being that everything is traced back to. Since the necessity of God would follow from these premises, then the denial of the existence of God would entail a contradiction.
Or, if we add the proposition that only the material world exists, and everything contingent can be traced back to a necessary entity, then it would follow either that the universe is a necessary entity or else some part of the universe is necessary. Since the necessity of the universe or some part of the universe would follow from these premises, then the denial of the existence of the universe would entail a contradiction. (Notice I didn't add "or some part of the universe." That's because if we deny the existence of the whole universe, then we would be denying the existence of every part of the universe as well.)
So what reason is there to think the universe might be contingent that wouldn't just as easily apply to God? Or what reason is there to think God is a necessary being that wouldn't just as easily apply to the universe? Even if we all agree that all contingent things have their origin in something that's non-contingent, we've got to figure out where we're going to halt our explanatory regress. And it can't be arbitrary. There's got to be a good reason for halting it where we halt it, whether we halt it with the universe or with something beyond the universe.
I've been thinking about that over the last two days, so I thought I'd share my thoughts with you. There are two things I have to say about it. First, I think it is plainly evident that anything composed of parts must be a contingent thing. The reason is because if something is composed of parts, then it can be disassembled. For example, my desk is composed of atoms. If those atoms were separated from each other and spread throughout the galaxy, then my desk would no longer exist.
However, it could be that while my desk, as a whole, is contingent, the individual parts that make up my desk might possibly be necessary. Ever since the pre-Socratic Greek philosophers, on down to modern day physicists, we have been trying to find out what the most fundamental thing in physical reality is. Many of the Greeks, like Lucretius and Democritus, said the most fundamental things are atoms. Fast forward a couple thousand years, and our physicists called the basic constituents of elements "atoms." But then we discovered that atoms are composed of protons, neutrons, and electrons. Then we discovered that protons and neutrons are composed of quarks. Now, some physicists think the most fundamental entities are tiny vibrating strings. Some think the most fundamental entities are quantum fields, like the electric field or the Higgs field. There's a field for every particle.
So while I can confidently say that atoms, stars, and people are contingent things, I've got to question whether or not the most fundamental thing (or things) that atoms, stars, and people are made of are also contingent things. If there is some fundamental, non-reducible, physical thing out of which all other physical things are composed, then whatever it is, it is not composed of divisible parts. It is simple in some sense.
That brings me to the second thing I had to say. Supposing the most fundamental things are strings or something yet undiscovered or even hypothesized, it would seem to be the case that there is some particular number of them. Maybe it's 10^500. Who knows? Whatever that number is, if we suppose these entities are non-contingent (i.e. they are necessary), it would follow that the number of them is also necessary. After all, if each one of them is necessary, then it's impossible for any one of them to not exist, and if it's impossible for any one of them to not exist, then there couldn't possibly be fewer of them. Also, if they are necessary, then there couldn't possibly be any more of them either. The reason is because there could only be more if more came into existence. But if something comes into existence, then there had to have been a state of affairs in which it didn't exist, in which case it couldn't have been a necessary thing. So if the fundamental particles, strings, or whatever of physical reality are necessary entities, then there is a fixed number of them, and whatever that number is, it is necessary that it be that number.
Doesn't that strike you as odd, though? That there would be some fixed number of strings, and that there couldn't possibly be one more or one less? While I can't prove it, I am inclined to think the number of fundamental particles/strings/whatever is not necessary. It is contingent. If it's even possible that there could be one more string or one less, then at least one of these strings is contingent. But if one is contingent, why think any of them at all would be necessary? How could we distinguish between contingent strings and non-contingent strings? If we suppose half of all strings are contingent, and the other half are non-contingent, then we're faced with the same issue. There would still be a fixed number of necessary strings.
So I have a hard time believing that any given number of things could be necessary. I have a much easier time believing that if something is necessary, then there's only one of its kind. It's easy for me to imagine that all contingent things have their origin in one necessary thing, but if you suppose that all contingent things have their origin in a multitude of things, say 5, 50, or 10^500, then I'd be left wondering why there has to be just that number and not one more or one less. I would be inclined to believe that the number is contingent, in which case we haven't traced everything back to something necessary yet. We would have to keep going until we traced everything back to one thing before I'd be satisfied.
It is hard for me to explain why I'd be satisfied with one necessary thing that is the source of everything else, but I'll try. We couldn't suppose that there was one less thing, because then there'd be nothing at all, and it wouldn't be the case that everything contingent is traced back to something non-contingent. Rather, everything would be contingent, and that doesn't even seem possible to me. But if we suppose there might two things rather than one, it's hard to see why "two" is a necessary number and why there couldn't be one more or one less. So one necessary thing is the only thing that really resonates with me. I don't know how to explain it any better than that.
If some physicist could show that the most fundamental physical reality is just quantum fields, and if that physicist could somehow show that there's only one quantum field (i.e. that the various fields we now suppose exist can somehow be unified), then I would be hard pressed to find any grounds to argue that the universe is a contingent thing. At that point, I'd probably be 50/50. I might wonder about the properties of the field, though, and whether those properties were necessary or contingent. It seems to me that for anything in motion, each state of that thing is contingent. But I don't think it would follow that the whole thing is contingent.
But supposing the universe is composed of irreducible strings or some other particles, then I would be strongly inclined to believe that the whole universe was contingent. And if the whole universe is contingent, then the explanation for the universe must be something beyond the universe. Now we're getting into God territory. Of course one can always suppose that the universe has it's origin in something else that is contingent, but even if so, that contingent thing would ultimately have to trace its origin to something non-contingent. You can't escape the existence of something non-contingent no matter how many contingent things you put before the universe. Whatever that thing is, and I'm strongly inclined to think it's just one thing for the reasons I've already given, it would have to be god-like. It would be a non-physical necessary being that is the origin of everything else that exists. I think we can attribute those properties at a minimum with a fair degree of certainty. We might be able to infer various other properties, but in that case I think we'd be on less firm ground and we'd be venturing into speculation.
So where do I stand on the argument from contingency? Well, I think the argument from contingency shows with a high degree of certainty that there is a necessary entity, probably just one, that is the source for everything else that exists. It falls short of certainty to the degree that brute facts are possible. This argument, by itself, doesn't show that YHWH exists, although the conclusion of this argument could be used as a premise in a larger argument for the existence of YHWH. For me, this argument could be strengthened if I had better reasons to think the universe is contingent. While I strongly suspect the universe is contingent, I don't really have a strong argument for it. Well, let me back up. I think Kalam-type arguments give us strong reasons to think the universe is contingent, but I'm trying to evaluate the argument from contingency as a stand-alone argument without invoking other theistic arguments to prop it up.