What's obscure about it? The Pawns of Null-A was his second best book (by my counting The Weapon Shops of Isher was the best). 😛

There's a problem with the term NULL-A in that book: non-Aristotelian is a pretty ambiguous (actually not ambiguous, because there are 3 possible meanings, not 2 :Whistling:) term which has been used to mean (a) disagreeing with Aristotle and accepting the principle of bivalence, (b) disagreeing with Aristotle and rejecting the principle of bivalence, and (c) agreeing with Aristotle but disagreeing with the idiots who use it to mean either (a) or (b) because they think that the principle of bivalence is the only thing that matters in logic. I guess Van Vogt's use of the term was with meaning (c), because he was a fan of Korzybski who was clearly a proponent of (c).

But of course the use of "NULL-a" (or, in some papers, just "a") in relational calculi has no real connection with its use by Van Vogt.