Buswell's Unusual Definition of "Presupposition"

In A Systematic Theology of the Christian Religion (Zondervan, 1962) J. Oliver Buswell Jr. presents an unusual definition of “presupposition.” He writes, “we take our presupposition as a conclusion arrived at on the basis of what we consider good and sufficient reasons.” (p. 15) This definition is unusual because it is essentially opposite of what everyone else means by “presupposition”! Presuppositions are usually thought of not as “a conclusion arrived at” but as “a beginning assumed.”
Buswell realized that his definition was unusual and so set to defending it:

“To the student who is not familiar with inductive processes of reasoning, the above paragraph may seem obviously absurd. If a presupposition is “a conclusion arrived at on the basis of . . .” then the presupposition is no presupposition, but the “basis” is really the presupposition. In answer I would argue that this objection confuses the chronological beginning, which may be anywhere, with the pedagogical beginning, which is selected for the purposes of exposition. The reader is reminded of the remarks in the preface, to the effect that the system of truth which we endeavor to proclaim is so thoroughly integrated that wherever one begins in systematic theology he must take for granted the other fundamentals of the system until they in turn can be investigated.”

I take it that Buswell’s “pedagogical beginning,” as it is “selected for the purposes of exposition,” must be the two presuppositions he had previously mentioned: “we presuppose the sovereign Triune God of the Bible” and “we presuppose the Bible as the infallible Word of God.” But, the “chronological beginning” according to Buswell can be anywhere. That is, because Christianity is a system of truth, one could start a systematic theology anywhere; by an exposition of Eschatology, the nature of man, or the nature of Christ.
But isn’t Buswell confused here? For these other possible “chronological beginnings” (eschatology, the nature of man, the nature of Christ, etc.) are INSIDE the system of Christianity; they are doctrines of the Scripture. Let us grant that holding to the belief of one of these doctrines of Scripture gives us good and sufficient reason to believe the other doctrines of Scripture. Yet, Buswell’s “good and sufficient reasons” often are from OUTSIDE the system of the Scriptures. For example, Buswell holds that the arguments for the existence of God (which are not fully given in the Scriptures) are inductive cumulative probabilistic arguments. That is, while none of the arguments are deductively valid proofs of God’s existence, each of the arguments, according to Buswell, add to the probability of there being a God. These arguments for the existence of God, for Buswell, are reasons for belief; perhaps even “good and sufficient reasons.”
One might quizzically ask, as Gordon Clark often did, “what is the probability of God’s existence based on those arguments?” That is, “what is the numerator and what is the denominator?” These arguments provide no definite probability. And in turn, they provide no “good and sufficient reason” for belief in God.
We cannot reason our way to the Triune God or the infallibility of the Scriptures from OUTSIDE of the system of Scriptures. We must hold, with the Westminster Confession of Faith, “The authority of the Holy Scripture, for which is ought to be believed, and obeyed, depends not upon the testimony of any man, or Church, but wholly upon God (who’s is the truth itself) the author thereof: and therefore is to be received, because it is the Word of God.”