While in discussion with A. C. Grayling on the March 25 edition of the Unbelievable radio program, Peter S. Williams provided a nice, concise presentation of the cosmological argument from contingency:
Once you’ve made the distinction between things that have causes and…things that don’t have causes, if something exists it either is the kind of thing that requires something outside of itself to exist, or it’s not. If it’s not possible for there to be an infinite regress of things that do require causes outside of themselves, and it is true that something exists which does require a cause outside itself [the universe, and everything in it]…,there can’t be an infinite regress of such causes, and therefore you have to have a termination of that regress. [God is the best explanation for the termination of that regress.]
For those of you for whom this to be a bit too concise, let me flesh it out a bit.
We can conceive of two different kinds of things: those whose existence requires a cause external to themselves (contingent beings), and those whose existence does not require a cause external to themselves (necessary beings). Given the fact that all physical things – the universe, and everything in it – did not have to exist, and at one point in time did not exist, we conclude that they are contingent beings whose existence requires a cause external to themselves.
The contingent nature of physical reality creates a problem for any naturalistic explanation of universe’s origin. To explain the existence of contingent being X, one must appeal to a prior being W that caused X to come into being; to explain the existence of contingent being W, one must appeal to a prior being V that caused W to come into being; to explain the existence of contingent being V, one must appeal to a prior being U that caused V to come into being, ad infinitum. To explain the existence of contingent being X, then, there would have to have been an infinite number of contingent beings preceding X that form a causal chain leading to the existence of X.
There are two problems with this. First, if there were an infinite regress of causes, it would actually make it impossible to explain the existence of X. Philosopher Richard Purtill offers a great analogy to illustrate this point. Imagine if I asked you to lend me a book. You say you don’t have it, but you’ll ask your friend if he has a copy he can lend to you, and in turn you’ll lend it to me. When you ask your friend for the book, he says he does not have it, but he’ll ask his friend if he has a copy to lend him. If so, he’ll lend it to your friend, who will then lend it to you. If this process continues ad infinitum, I will never receive the book. Likewise, if no contingent being in the infinite causal chain that leads to X has existence in itself, then no contingent beings could ever exist since contingent beings must derive their existence from a source external to themselves. To assert that contingent beings could exist apart from a source who possesses being in and of itself is like suggesting that an infinite number of boxcars in an infinitely long train can be in a state of motion, despite the fact that there is no engine to pull them. If there is no engine to pull car A, then car A and every car attached to A would never begin to move. The fact that the train is moving demonstrates that at least one car has an engine capable of giving movement to all the other cars. Likewise, the fact that contingent being X exists demonstrates that there is some non-contingent being in the causal series that gives being to all other beings.
Second, we know from scientific discoveries that physical reality has a finite past. The causal chain that led to X terminates with the singularity. There are no physical entities, causes, or events beyond the singularity, and thus there is no infinite regress of external causes that one can appeal to that explains the existence of X. Contingent being W can only explain contingent being X if there is a contingent being V that can explain contingent being W, and contingent being V can only explain contingent being W if there is a contingent being U that can explain contingent being V, and so on. But given the temporal finitude of the universe, we will ultimately arrive at contingent being A which explains contingent being B, .but what can explain contingent being A? It requires a cause external to itself in the same manner as all other contingent beings, but as the first contingent being there simply aren’t any other contingent beings available to explain A. If contingent beings derive their existence from a source external to themselves, and there is no contingent being external to A to explain the existence of A, then the existence of X cannot be explained either. We must either conclude that there is no explanation of contingent beings (in violation of the principle of sufficient reason), or that reality is not be exhausted by physical reality. Given the strength of our metaphysical intuition that being only comes from being, the latter is more reasonable. In addition to the contingent beings that constitute physical reality, there must also exist a necessary being that transcends the physical realm.
Going back to Richard Purtill’s book-borrowing analogy for a moment, we said that if the process of requesting the book from a friend went on ad infinitum, I would never receive the book. If I do receive the book, however, then somewhere down the causal chain there must exist someone who does not have to borrow the book, but owns the book and lends it to all others who need the book. Similarly, if contingent being X exists, then somewhere down the causal chain there must exist a necessary being that does not have to derive its existence from some external source, but exists by a necessity of its own nature, and is the source of all contingent beings. Such a being cannot be part of the physical realm, because all physical entities are contingent beings. The necessary being must transcend the physical realm, acting as its first cause.
In addition to immateriality, the necessary being must also be eternal and nonspatial since time and space are part of physical reality. A being with such characteristics matches what theists have traditionally described as God, and thus God is the best candidate for the necessary being.
In summary, there cannot be an infinite regress of things that require a cause external to themselves. There must be a termination of the causal chain. Whatever terminates the causal chain cannot itself be something that requires a cause external to itself, but must be a necessary being from which all contingent beings derive their existence. Given the characteristics of a necessary being, God is the best explanation for the termination of the causal regress.
Richard Purtill, Contemporary Philosophy of Religion (Blackwell, 2001), 358-359. Quoted in http://www.bethinking.org/science-christianity/advanced/a-universe-from-someone-against-lawrence-krauss.htm.