Van Til claims that “the method of Christian theism [is] the method of implication” (Introduction, p27). By this, Van Til means a combination of a priori and a posteriori approaches to systematic theology. Putting things simply, a priori knowledge is independent of experience (literally:”from what is before”), while a posteriori knowledge depends upon or is drawn from experience (literally: “from what is after”).
In this context, “a priori” refers to the body of facts received by revelation and which is therefore prior to experience. The truths of Scripture are first revealed by God to Man, and only then encountered by Man. There are many more facts available to us, but those contained in Scripture are not dependent upon nor drawn from our own experience.
Secondly, the Christian method possesses an a priori element since we interpret the facts of nature and experience in the light of a priori revelation:
“It may be admitted that the truths which the theologian has to reduce to a science, or, to speak more humbly, which he has to arrange and harmonize, are revealed partly in the external works of God, partly in the constitution of our nature, and partly in the religious experience of believers; yet lest we should err in our inferences from the works of God, we have a clearer revelation of all that nature reveals, in his word; and lest we should misinterpret our own consciousness and the laws of our nature, everything that can be legitimately learned from that source will be found recognized and authenticated in the Scriptures; and lest we should attribute to the teaching of the Spirit the operations of our own natural affections, we find in the Bible the norm and standard of all genuine religious experience.” (Hodge, Systematic Theology, 1:12 [Ages Software, 2005])
On the other hand, the Christian method also has an a posteriori element. This refers to the “gathering and arranging of the facts of Scripture” (Introduction, p27). Hodge says such a collection must be i) conducted with diligence and care ii) be “comprehensive and, if possible, exhaustive” since:
“An imperfect induction of facts led men for ages to believe that the sun moved round the earth, and that the earth was an extended plain. In theology a partial induction of particulars has led to like serious errors.” (ibid, 1:13)
After all, it will be impossible to defend error.
Van Til continues by arguing that it is not enough for the Christian method to possess both an a priori element and an a posteriori element, since some flavours of idealism also embraces both. He then goes on to critique Idealism, which I will save for another time.