A presupposition is not a belief that one must have before (temporally speaking) one comes to believe in other things; rather, it is a belief that is independent of some other knowledge and governs that knowledge to some extent.

John Frame, Cornelius Van Til: An Analysis of His Thought (P&R, 1995), 132 n5.

Elsewhere, Van Til ably demonstrates that EVERYONE has presuppositions which govern their thinking. A major task of apologetics is exposing the presuppositions of unbelievers (because they often keep them hidden, or are unaware of them), and demonstrating them to ultimately be irrational (which all presuppositions apart from Christian theism are).