"The epistemological and metaphysical principles that underlie and control one's method." (Van Til, 312) Every method, says Van Til, even neutral ones presuppose either the truth of or the falsity of Christian theism. You can't appeal to "facts" and "laws" because no common definitions exist. To quote Van Til himself:
The theistic proofs therefore reduce to one proof, the proof which argues that unless this God, the God of the Bible, the ultimate being, the Creator, the controller of the universe, be presupposed as the foundation of human experience, this experience operates in a void. This one proof is absolutely convincing. (Van Til, 313)
This is quite different to Descartes' starting point "I think, therefore I am." There are no neutral or abstract starting points so it's better as Van Til argues to start outside yourself with God. Everyone has a starting point for either God or unGod. Everything we believe as Christians is built on this presupposition.
Evangelism should says Van Til, be built on the friendly assumption that for awhile you and your interlocutor will adopt each other's points of view "for the sake of argument." ( The goal is to create some sort of common ground or shared definition.) Then in that adopted framework you reduce the beliefs down to that essential presupposition; God or unGod.
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