25 January 2022, The Utrecht Logic in Progress Series (TULIPS), Stipe Pandzic

Abstract: In the 1980s, John Pollock’s work on defeasible reasons started the quest in the AI community for a formal system of defeasible argumentation. My goal in this talk is to present a logic of structured defeasible argumentation using the language of justification logic. One of the key features that is absent in standard justification logics is the possibility to weigh different epistemic reasons that might conflict with one another. To amend this, we develop a semantics for “defeaters”: conflicting reasons forming a basis to doubt the original conclusion or to believe an opposite statement. Formally, non-monotonicity of reasons is introduced through default rules with justification logic formulas.

Justification logic formulas are then interpreted as arguments and their acceptance semantics is given in analogy to Dung’s abstract argumentation framework semantics. In contrast to argumentation frameworks, determining arguments’ acceptance in default justification logic simply turns into finding (non-monotonic) logical consequences from a starting theory with justification assertions. We can show that a large subclass of Dung’s frameworks is a special case of default justification logic. By the end of the talk, I will briefly connect this logic with Pollock’s original motivation to deal with the justified-true-belief definition of knowledge.