A redated post.
Perhaps the most enduring of Aquinas's Five Ways has been the argument from contingently existing things, the third way. Amazingly, some people actually think that Aquinas's cosmological argument is something like this.
1. Everything has a cause.
2. So the Universe has a cause.
Therefore, God exists.
Now let's set aside the question of whether that which causes the universe has to be God. Surely there is something God-like about anything that has the power to cause the universe to exist, even if that being is not, strictly speaking, God. At the very least, naturalism, the view that they there is nothing over and above the physical universe, would be false if this argument were to be correct.
But the problem is obvious. If the universe has to have a cause because everything has to have a cause, then the universe not only has a cause, it has a cause of its cause, and a cause of the cause of the cause, and the cause of the cause of the cause of the cause....There are people, Like Bertrand Russell, who have suggested that you can refute Aquinas, or even all forms of the cosmological argument, by asking the question that an 8-year-old child knows how to ask, namely "Who made God?" Aquinas may have been called the Dumb Ox, but you don't get to be the Angelic Doctor by being stupid. I was a little surprised to find an article, published in Philo in 1998, which attributed this kind of argument to Aquinas.
The range of what needs a cause, in other words, has to be restricted, so that the things in the physical world do need causes, and God does not need a cause. One way to do that it in the way that is employed by the present-day Kalam Cosmological Argument:
1. Whatever begins to exist, must have a cause of its existence.
2. The universe began to exist.
Therefore, the universe has a cause of its existence.
This argument doesn't require that God needs a cause, because God never began to exist. However, Aquinas did not argue in this way either. The reason for this is that Aristotle had said that even though the universe is caused to exist by the Unmoved Mover, it nonetheless never began to exist. It has existed from eternity. And Aquinas maintained that while you couldn't prove Aristotle right on this score, you couldn't prove him wrong either, and so premise 2 is an article of faith rather than an article of reason; that is something that is must be believed on the basis of the revelation delivered through the Bible and the Church. (Aquinas' concept of faith is not what a lot of people today mean by faith; belief that is contrary to reason). But articles of faith are going to be useless in arguments for the existence of God, since obviously anyone you are trying to persuade will not accept the Bible and the Church as authorities. (You may be able to use the Bible as ancient historical documentation, but not as the Word of God).
So how did St. Thomas Aquinas argue for theism? Peterson, Hasker, Reichenbach, and Basinger, in their book Reason and Religious Belief (OUP, 1991) present the following argument, which they call the Thomistic cosmological argument.
1. A contingent being exists.
2. This contingent being has a cause of its existence.
3. The cause of its existence is something other than itself.
4. What causes this contingent being to exist must be a set that contains either only contingent beings or a set that contains at least one noncontingent (necessary) being.
5. A set that contains only contingent beings cannot cause this contingent being to exist.
6. Therefore, what causes this contingent being must be a set that contains at least one necessary being.
7. Therefore, a necessary being exists.
Here, we can respond to any challenge coming from the "Who made God" quarter by pointing out that a the causal principle in 2 requires only that contingent beings need a cause for their existence.
The critical premise here seems to be 5. One could argue, and I that Aquinas does argue, that if the series is a series of contingent beings, then there would have to be an infinite number of contingent beings. But the number of contingent beings in the universe is finite, so this can't be right.
Further, if there were an infinite number of contingent beings, each explanation would be an explanation in terms of something that needs an explanation as much as did the thing we were explaining. If you explain the position of the earth by saying it rests upon a turtle, is it really a satisfying answer to say that it's turtles all the way down.
We might want to ask this question: Why are there any contingent beings at all? The existence of particular contingent things that explain the existence of particular other things is not a satisfying answer to this question.
Russell responded to this question by saying "The universe is just there and that's all." If everything in the universe is contingent, does that make the universe contingent. But if each particle in the universe can cease to exist, doesn't it make sense to suggest that they all can cease to exist collectively?
But how do we know that everything in the physical world is contingent in the required sense? I believe that my car exists contingently because I have seen cars fall apart and go to the junkyard. I believe that people and animals exist contingently because I have seen them die. But what happens when I die? Since this is a theistic argument, we have to assume that our discussion partner is an atheist. What an atheist most likely believes is that we are all conglomerations of material particles. While we are alive, those particles work together organically, when we die, the functional unity that exists amongst the particles in our bodies is dissolved.
But while we know that organized unities of things in the world invariably fall apart, do we have equal confidence that the basic particles we find in the world (atoms, quarks, strings, or whatever they finally tell us it is) exist contingently. Scientists actually tell us that matter is not created, and matter is not destroyed. So couldn't the basic stuff of the physical universe be the thing that exist non-contingently, the existence of which does not depend on anything else for its existence.
So, does the basic stuff of the physical world exist contingently? I can't find anything in Aquinas that gives me a reason to think that it does. But suppose we have good reason to suppose that the universe began to exist. This was an article of faith for Aquinas, but if the standard interpretation of Big Bang cosmology is correct, then it has a basis in modern science. Historically, the atheist has answered the question "But why does the matter in the universe exist" by answering that the matter in the universe has always been there, and since it has always been there its existence does not need to be explained. If this response isn't available, then doesn't the beginning of the universe at the Big Bang provide a reason for supposing that the stuff of the physical universe exists contingently and not non-contingently?