ABSTRACT: Justification Logic extends the usual modal logic of knowledge and belief with a component representing justification. The celebrated account of knowledge as “justified true belief,” which is attributed to Plato, has long been a focus of epistemic studies. About a half-century ago, the notions of knowledge and belief acquired formalization by means of modal logic. However the notion of justification, an essential element of epistemic studies, was conspicuously absent, and this led to well-known deficiencies inherent in modal logics of knowledge.

Justification Logic extends the logic of knowledge in three major ways. First, it adds a long-anticipated mathematical notion of justification, making the logic more expressive. We now have the capacity to reason about justifications, simple and compound. We can compare different pieces of evidence pertaining to the same fact. We can measure the complexity of justifications, thus connecting the logic of knowledge to a rich complexity theory, etc. Second, justification logic furnishes a new, evidence-based foundation for the logic of knowledge, according to which F is known' is interpreted as F has an adequate justification.’ Third, justification logic provides a novel, evidence-based mechanism of truth tracking which can be a valuable tool for extracting robust justifications from a larger body of justifications which are not necessarily reliable.