It appears then that if there is to be any intelligible encounter between the Christian and the non-Christian, it must be in terms of the two mutually exclusive visions that each entertains. To appeal to the law of contradiction and/or to facts or to a combination of these apart from the relation that these sustain to the totality-vision of either, the believer or the unbeliever, is to beat the air. It is well to say that he who would reason must presuppose the validity of the laws of logic. But if we say nothing more basic than this, then we are still beating the air. The ultimate question deals with the foundation of the validity of the laws of logic. We have not reached bottom until we have seen that every logical activity in which any man engages is in the service of his totality-vision.
It is also well to say that we must follow the facts wheresoever they may lead us. But again we should note that all research into the realm of fact, on the part of any man, is in the service of his totality-vision. The self-authenticating man assumes that if the Christian story were true, then the scientific enterprise would be meaningless. Free scientific inquiry, he assumes, requires that there be no pre-interpretation of facts in terms of the Christian story. On the other hand, the Christian holds that the idea of free scientific inquiry is unintelligible except upon the presupposition of the truth of the Christian story.
– Cornelius Van Til, The Case for Calvinism p.137