A logical expression of reasoning

We introduce a new nonmonotonic logic, the Logic of Plausible Reasoning, LPR, capable of dealing with creative complex reasoning, which is argued to be the kind of reasoning required in many instances of scientific thought, professional practice and common sense decision taking. For managing the simultaneous consideration of multiple scenarios inherent in these activities, two new modalities, weak and strong plausibility, are introduced as part of a monotonic logic specially designed to support LPR reasoning, which is called Logic of Plausible Deduction, LPD. Axiomatics and semantics for LPD, together with a completeness proof, are provided. Once LPD is given, LPR may be defined via a refined concept of extension over LPD. Although the construction of LPR extension is first presented in standard style, for the sake of comparison with existing nonmonotonic logics, alternative more elegant ways for constructing nonmonotonic LPR extensions are also given and proofs of their equivalence are provided.