Arthur Pap was not quite a Logical Empiricist. He wrote his dissertation in philosophy of science under Ernest Nagel, and he published a textbook in the philosophy of science at the end of his tragically short career, but most of his work would be classified as analytic philosophy. More important, he took some stands that went against Logical Empiricist orthodoxy and was a persistent if friendly critic of the movement. Pap diverged most strongly from Logical Empiricism in his theory of a “functional a priori” in which fundamental principles of science are hardened into definitions and act as criteria for further inquiry. Pap was strongly influenced by the pragmatists C. I. Lewis and John Dewey in developing this alternative theory of a priori knowledge. Using Poincaré’s conventionalism as a springboard, Pap attempted to substantiate these views with examples from physics, and this was his largest foray into philosophy of science topics. Pap, as well as Lewis and Dewey, developed an alternative theory of the a priori in the 1950s that never quite took hold, despite the fact that their views are very intriguing and similar to Michael Friedman’s recent work on the constitutive a priori.
Stump, David J., "Arthur Pap’s Functional Theory of the A Priori" (2011). Philosophy. 15.