Throughout his philosophical career, Hilary Putnam was preoccupied with the question of what survives of the traditional notion of a priori truth in light of the recurring historical phenomenon, made prominent by the scientific revolutions of the early decades of the twentieth century, through which “something that was literally inconceivable has turned out to be true”. Impugning the analytic-synthetic dichotomy, Putnam’s redefinition of “conceptual truth” in terms of “quasi-necessity relative to a conceptual scheme” is meant to accommodate the possibility of (...) transitions of just this sort. In this essay, I trace the origins and development of Putnam’s account of “quasi-necessity.” I try to defend it against some objections naturally arising in connection with the interplay of modality and negation. My main contention is that the main tenets of Putnam’s semantic externalism inform his reconception of conceptual truth, and that they must be recognized to hold of such basic logical notions as those of judgment and negation. (shrink)