Principles and Examples of Plausible Reasoning and Propositional Plausible Logic

Abstract

Plausible reasoning concerns situations whose inherent lack of precision is not quantified; that is, there are no degrees or levels of precision, and hence no use of numbers like probabilities. A hopefully comprehensive set of principles that clarifies what it means for a formal logic to do plausible reasoning is presented. A new propositional logic, called Propositional Plausible Logic (PPL), is defined and applied to some important examples. PPL is the only non-numeric non-monotonic logic we know of that satisfies all the principles and correctly reasons with all the examples. Some important results about PPL are proved.

58 pages. Updated n-die examples to n-lottery examples. Example 3.4 simplified. In Section 4 counter-example to the Or-rule changed and corrected, and a new logic (PTL) considered