A “presupposition” is an elementary assumption in one’s reasoning or in the process by which opinions are formed. … a “presupposition” is not just any assumption in an argument, but a personal commitment that is held at the most basic level of one’s network of beliefs. Presuppositions form a wide-ranging, foundational perspective (or starting point) in terms of which everything else is interpreted and evaluated. As such, presuppositions have the greatest authority in one’s thinking, being treated as one’s least negotiable beliefs and being granted the highest immunity to revision.
Source: Greg Bahnsen, Van Til’s Apologetic: Readings & Analysis (Phillipsburg: Presbyterian and Reformed Publishing, 1998) page 2, footnote 4