April 21, 2021

Abstract: It is straightforward to treat the identity predicate in models for first order predicate logic. Truth conditions for identity formulas are straightforward. On the other hand, finding appropriate rules for identity in a sequent system or in natural deduction leaves many questions open. Identity could be treated with introduction and elimination rules in natural deduction, or left and right rules, in a sequent calculus, as is standard for familiar logical concepts. On the other hand, since identity is a predicate and identity formulas are atomic, it is possible to treat identity by way of axiomatic sequents, rather than inference rules. In this talk, I will describe this phenomenon, and explore the relationships between different formulations of rules for the identity predicate, and attempt to account for some of the distinctive virtues of each different formulation.


I’m Greg Restall, and this is my personal website. I teach philosophy and logic as Professor of Philosophy at the University of Melbourne. ¶ From August 2021, I will be the Shelby Cullom Davis Professor of Philosophy at the University of St Andrews. ¶ Start at the home page of this site—a compendium of recent additions around here—and go from there to learn more about who I am and what I do. ¶ This is my personal site on the web. Nothing here is in any way endorsed by the University of Melbourne.



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