Protopopescu: Discovering Knowability

by Yang Liu

CUNY Computational Logic Seminar
September 4, Time 2:00 – 4:00 PM, Room 3309.
Speaker: Tudor Protopopescu (Graduate Center)
Title: Discovering Knowability: A Semantic Analysis (a joint work with S. Artemov, just appeared in Synthese)

Abstract: We provide a semantic analysis of the well-known knowability paradox stemming from the Church-Fitch observation that the meaningful knowability principle all truths are knowable, when expressed as a bi-modal principle F \to \diamond KFyields an unacceptable omniscience property all truths are known. We offer an alternative semantic proof of this fact independent of the Church-Fitch argument. This shows that the knowability paradox is not intrinsically related to the Church-Fitch proof, nor to the Moore sentence upon which it relies, but rather to the knowability principle itself. Further, we show that, from a verifiability perspective, the knowability principle fails in the classical logic setting because it is missing the explicit incorporation of a hidden assumption of stability: `the proposition in question does not change from true to false in the process of discovery.’ Once stability is taken into account, the resulting stable knowability principle and its nuanced versions more accurately represent verification-based knowability and do not yield omniscience.