Adapted from Van Til Diagrammed.
Hume said that all knowledge arises from experience. Kant replied that we should also accept that which is necessary for the possibility of experience, even though it is never experienced: "But though all of our knowledge begins with experience, it does not follow that it all arises out of experience" (Critique of Pure Reason). This type of argument is called a "transcendental argument." Kant's specific argument is that the autonomous human mind imposes order on experience to make experience intelligible. Van Til adopts Kant's transcendental type of argument, but rejects his specific claim, that the autonomous human mind can serve as a basis for intelligible experience. Rather, starting with the autonomous, sovereign, self-sufficient Creator who is the source of both the unity and diversity of experience is necessary to account for the possibility of intelligible experience. But by "starting point" here, Van Til means the ultimate source of knowledge, that which must be presupposed to account for the possibility of knowledge. God is the ultimate starting point of knowledge. But he says that God is not necessarily the proximate starting point. We can deduce the necessity of God as the ultimate starting point from examining our proximate starting point - the facts of our experience. That should lead us to conclude that God exists.
We cannot be too careful about asking what the starting point of any one’s argument is. It is of the utmost importance that we find our way through the maze of confusion that prevails on this subject.
As a help to clarification of this subject we may perhaps suggest a distinction between an immediate and an ultimate starting point. By an immediate starting point is meant the place where the knowledge of facts must begin. It is of course quite consistent with a theistic position to say that we must start with the “facts” as that term is understood ordinarily. Neither Augustine nor Calvin would have objected to saying that knowledge of self was their immediate and temporary starting point. But when the question of an ultimate starting point is raised the matter is different. In that case Augustine and Calvin would both have to say that their ultimate starting point is God. That is, they could intelligently think of their own non-existence but were unable to think intelligently of God’s nonexistence. The difference may perhaps be brought out by the analogy of a diving board. Suppose a diver was standing on the tip of a diving board and that all that he could see of the diving board was the very tip on which he was standing. Suppose further that all that he could see around him was water. Now if he should say that the very spot from which he was about to make his leap is his starting point he might mean either of two things. If we thought of him as unaware of the connection of the point on which he was standing with the foundation on which it rested he would be speaking of that particular spot as the permanent or ultimate starting point. On the other hand, if he were fully aware of the fact that the tip of the diving board is only a tip of a board that rests upon a solid rock under water, he might speak of that tip as a starting point but only as an immediate starting point. The real and ultimate starting point for him would be the foundation on which the whole diving board was resting. Similarly we may say that the question at issue is not that of what is the immediate starting point. All agree that the immediate starting point must be that of our everyday experience and the “facts” that are most close at hand. But the charge we are making against so many Idealists as well as Pragmatists is that they are taking for granted certain temporal “facts” not only as a temporary but as an ultimate starting point. . . . Yet the very point in question is whether any statement can be made about any appearance at all without reference to the fact of God."
A Survey of Christian Epistemology (Phillipsburg, NJ: The Presbyterian and Reformed Publishing Company, 1969), pp. 119-21.