A logic of default justifications

We define a logic of default justifications that relies on op- erational semantics. One of the key features that is absent in standard justification logics is the possibility to weigh differ- ent epistemic reasons or pieces of evidence that might con- flict with one another. To amend this inadequacy, we develop a semantics for “defeaters”: conflicting reasons forming a ba- sis to doubt the original conclusion or to believe an opposite statement. Our logic is able to address interactions of normal defaults without relying on priorities among default rules and introduces the possibility of extension revision for normal de- fault theories.