There is an odd artifact I've noticed in discussions between followers of Van Til and those of Clark. For some reason, those in the camp of Van Til take great delight in pointing out that Clark used the term "know" to refer to what we would call "know with absolute certainty." As such, Clark did not "know" that the woman with whom he was living was his wife. Endless merriment such comments make, particularly when the quotation marks around "know" are removed!
But why? Is it just to goad on the followers of Clark? Is it simply for the pleasure of hearing the sound of "you don't know that I am even real" or is there a deeper reason?
Surely the reason cannot be that the followers of Van Til think that Clark was wrong, and that Clark could know with absolute certainty that the woman he was living with was his wife. After all, it's imaginable that his parents-in-law had identical twins, one of whom was given up at birth. By chance, this twin sister discovered her long separated twin, murdered her in a jealous rage, and took her place. We could think of even more implausible options, but this relatively simple account provides one way that a person might be mistaken about such an important issue. Is it probable? No, it's not (though, of course, Clark was justifiably uncomfortable with such a concept), but the issue is certainty, not probability.
In the end, Clark is right in saying that the only things we can know with absolute certainty are those things that are revealed to us by God (whether through general or through special revelation). The only way to be absolutely sure about something is to obtain that knowledge from an absolutely reliable source.