Comparing truth theories based on Weak and Strong Kleene logic

Abstract

We compare the conceptual strength of two axiomatic theories of truth: KF and WKF. The first one has been designed to capture Kripke's fixed-point construction based on Strong Kleene logic. The second one is based on the Weak Kleene evaluation schema. In the paper "Relative truth definability of axiomatic truth theories", Kentaro Fujimoto proved that WKF is relatively truth-definable in KF. However, it has been an open question whether KF is relatively truth-definable in WKF. We provide the negative answer to this question, one that does not depend on the choice of language and coding. We consider this remarkable, because various important properties of Weak Kleene fixed-point construction are not absolute in this sense.