Suppose I ask “why does (a) exist?” or “what is the explanation for the existence of (a)?”—an explanation here constituting a reason whereby we logically understand a thing’s existence--and I receive the answer that (b) provides the explanation for (a). A subsequent question could be posed, “what is the explanation for (b)?”, and if the explanation for (b) is (c), then it is easy to see that we have the making of a potential infinite regress of explanations—(a) is explained by (b), which is explained by (c), on and on ad infinitum. However, an infinite chain of explanation seems logically suspect, but whether or not it is indeed suspect need not concern us here.I believe that most would agree that an explanatory chain does in fact end somewhere. The question that is important is where exactly does it end? A naturalist would claim that it ends with the universe, while a theist would claim that it ends with God. From this perspective it might seem that both explanatory chains end in a brute fact—something that just exists for no reason, with no explanation. Therefore, it might appear that each worldview, naturalism and theism, must admit of some brute fact that eludes explanation.
However, I maintain that this is not the case. For it is also possible—and I would argue, this is not only possible, but necessary--that the explanatory chain ends because the last member of the chain is self-explanatory. And if the last member of an explanatory chain is self-explanatory, then it is not a brute fact at all, because this member does in fact have an explanation—remember that an explanation here means a reason whereby we understand or make sense of some aspect of a thing’s being--for its existence. (Let it be known that this is exactly what the Principle of Sufficient Reason stipulates—namely, that that everything that exists either (1) has an explanation for its existence outside of itself, or (2) has an explanation for its existence contained within itself. So, the last member of an explanatory chain cannot be a brute fact, but must be self-explanatory. But, since the PSR is not crucial here, I will abstain from defending it presently.)
Now remember that, in the classical theistic tradition, God is such that his essence just is existence, because he is necessary and cannot not exist. This means that the proposition “God must exist” is a self-evident truth, since by “God” we mean “existence itself”. Thus, the proposition really means “that which is existence itself must exist” and it should be clear that this is self-evidently true.
I maintain that if the proposition “(x) must exist” is self-evident, then we can say that (x) is self-explanatory. For the self-evidence of the above proposition entails that the essence of (x) contains its existence—that is, it is of the nature of (x) that it must exist—and, therefore, the explanation for (x)’s existence lies within itself. Contrarily, if “(x) must exist” is not self-evident then (x) requires an explanation for its existence outside of itself, and is therefore not self-explanatory. (It should be clear that the proposition “the universe must exist” is not self-evident, and therefore the universe cannot be the last member of an explanatory chain.) Therefore, since “God exists” is self-evident, we have warrant to conclude that the claim of self-explanation must be applied to God. Thus, the explanatory chain ends in God, and does not end in a brute fact.
So, if God is indeed the last member in a chain of explanation, then asking “why does God exist?” will entail an answer already inherent in the concept of God. That is, the answer will be something along the lines of “God exists because he exists”—which sounds identical to God’s claim in the Bible: I am that I am. This is exactly what a self-explanatory essence entails.