Ultimate Presupposition
The natural man will invariably employ the tool of his reason to reduce these contents to a naturalistic level. He must do so even in the interest of the principle of contradiction. For his own ultimacy is the most basic presupposition of his entire philosophy. It is upon this presupposition as its fulcrum that he uses the law of contradiction. If he is asked to use his reason as the judge of the credibility of the Christian revelation without at the same time being asked to renounce his view of himself as ultimate, then he is virtually asked to believe and to disbelieve in his own ultimacy at the same time and in the same sense.
Cornelius Van Til, The Defense of the Faith (The Presbyterian and Reformed Publishing Company: Philadelphia, 1955).