 One way in which a priori knowledge of a logical truth such as (1) would be possible would be if a priori knowledge of the fact that (1) is a logical truth, or of the universal generalization “For all suitable \(a\), \(P\), \(b\) and \(Q\), if \(a\) is \(P\) only if \(b\) is \(Q\), and \(a\) is \(P\), then \(b\) is \(Q\)” were possible. One especially noteworthy kind of skeptical consideration in the epistemology of logic is that the possibility of inferential a priori knowledge of these facts seems to face a problem of circularity or of infinite regress. If we are to obtain inferential a priori knowledge of those facts, then we will presumably follow logical rules at some point, including possibly the rule of modus ponens whose very correctness might well depend in part on the fact that (1) is a logical truth or on the truth of the universal generalization “For all suitable \(a\), \(P\), \(b\) and \(Q\), if \(a\) is \(P\) only if \(b\) is \(Q\), and \(a\) is \(P\), then \(b\) is \(Q\)”. In any case, it seems clear that not all claims of this latter kind, expressing that a certain truth is a logical truth or that a certain logical schema is truth-preserving, could be given an a priori inferential justification without the use of some of the same logical rules whose correctness they might be thought to codify. The point can again be reasonably derived from Carroll (1895). Some of the recent literature on this consideration, and on anti-skeptical rejoinders, includes Dummett (1973, 1991) and Boghossian (2000).