An approach to Christian apologetics that argues the Christian worldview — the Triune God, the authority of Scripture, the reality of sin, salvation through Christ — must be presupposed as the necessary foundation for making sense of anything, including logic, science, morality, and meaning. Developed in its modern form by Cornelius Van Til (1895-1987) of Westminster Theological Seminary, extended by Greg Bahnsen, John Frame, and K. Scott Oliphint. Contrasts with evidentialism (which marshals evidence to show Christianity probably true) and classical apologetics (which uses traditional arguments for God's existence first, then moves to Christ).
Presuppositionalism has four distinctive commitments. (1) No neutral ground. Van Til insisted there is no "neutral" standpoint from which believers and unbelievers can argue. Every person interprets reality through ultimate commitments. Starting with "evidence" supposedly neutral actually smuggles in non-Christian assumptions (that the human mind can judge truth independently, that sense experience is reliable, that logic is universal). Presuppositionalism begins with the Christian presuppositions openly. (2) Transcendental argument. The presuppositional argument is transcendental — arguing that Christianity is the necessary precondition for the intelligibility of any experience whatsoever. Laws of logic, uniformity of nature, objective morality, and reliability of reason all require God as their ground. Bahnsen summed up: "The proof of God is that without Him you could not prove anything." (3) Scripture as the starting point. Where classical apologetics attempts to reason up to Christianity, presuppositionalism starts from Christian revelation and shows that all competing systems collapse into incoherence. (4) Criticisms and refinements. Critics charge presuppositionalism with circular reasoning; defenders respond that all ultimate worldviews are "circular" at the ultimate level (what is the ultimate justification for any system?), and the Christian circle is uniquely coherent. Proponents acknowledge evidential arguments can support presuppositional commitments. In practice, most apologists use a mix of approaches. Schaeffer's L'Abri, Frame's "multi-perspectival" approach, and Tim Keller's synthesis all move toward combining the best of each tradition.