The Scandinavian Logic Society

NOL Seminar with Melvin Fitting

The Nordic Online Logic Seminar is organised monthly over Zoom, with expository talks on topics of interest for the broader logic community. The seminar is open for professional or aspiring logicians and logic aficionados worldwide. If you wish to receive the Zoom ID and password for it, as well as further announcements, please subscribe here: https://listserv.gu.se/sympa/subscribe/nordiclogic.

Next talk: Monday, October 24, 16.00-17.30 CET (UTC+2), on Zoom (details are provided to the seminar subscribers)

Title: Strict/Tolerant Logic and Strict/Tolerant Logics

Speaker: Melvin Fitting, Professor Emeritus at City University of New York (Graduate Center)

Strict/tolerant logic, ST, has been of considerable interest in the last few years, in part because it forces consideration of what it means for two logics to be different, or the same. And thus, of what it means to be a logic. The basic idea behind ST is that it evaluates the premises and the conclusions of its consequence relation differently, with the premises held to stricter standards while conclusions are treated more tolerantly. More specifically, ST is a three-valued logic with left sides of sequents understood as if in Kleene’s Strong Three Valued Logic, and right sides as if in Priest’s Logic of Paradox. Surprisingly, this hybrid validates the same sequents that classical logic does, though it differs from classical logic at the metaconsequence level. A version of this result has been extended to meta, metameta , etc. consequence levels, creating a very interesting hierarchy of logics. All this is work of others, and I will summarize it.

My contribution to the subject is to show that the original ideas behind ST are, in fact, much more general than it first seemed, and an infinite family of many valued logics have Strict/Tolerant counterparts. Besides classical logic, this family includes both Kleene’s and Priest’s logics themselves, as well as first degree entailment. For instance, for both the Kleene and the Priest logic, the corresponding strict/tolerant logic is six-valued, but with differing sets of strictly and tolerantly designated truth values. There is a reverse notion, of Tolerant/Strict logics, which exist for the same structures. And the hierarchy going through meta, metameta, … consequence levels exists for the same infinite family of many valued logics. In a similar way all this work extends to modal and quantified many valued logics. In brief, we have here a very general phenomenon.

I will present a sketch of the basic generalizations, of Strict/Tolerant and Tolerant/Strict, but I will not have time to discuss the hierarchies of such logics, nor will I have time to give proofs, beyond a basic sketch of the ideas involved.