Plausible Reasoning and First-Order Plausible Logic

arXiv cs.AI / 4/22/2026

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Key Points

The paper introduces "plausible reasoning" as a form of non-numerical reasoning that draws conclusions from factual and defeasible (likely-but-not-certain) statements without using probabilities.
It proposes seventeen principles for logics that support plausible reasoning, including fourteen necessary and three desirable principles, and notes that one desirable principle is not formally stated.
A first-order logic called Plausible Logic (PL) is defined that satisfies all but two of the desirable principles and is claimed to correctly handle the provided examples.
The authors present a set of reasoning algorithms for PL (eight total) to account for different reasonable conclusions that can arise from the same plausible reasoning situation.
This work is a condensed version of the author’s forthcoming book, with proofs deferred to that book.

Abstract

Defeasible statements are statements that are likely, or probable, or usually true, but may occasionally be false. Plausible reasoning makes conclusions from statements that are either facts or defeasible statements without using numbers. So there are no probabilities or suchlike involved. Seventeen principles of logics that do plausible reasoning are suggested and several important plausible reasoning examples are considered. There are 14 necessary principles and 3 desirable principles, one of which is not formally stated. A first-order logic, called Plausible Logic (PL), is defined that satisfies all but two of the desirable principles and reasons correctly with all the examples. As far as we are aware, this is the only such logic. PL has 8 reasoning algorithms because, from a given plausible reasoning situation, there are different sensible conclusions. This article is a condensation of my book `Plausible Reasoning and Plausible Logic' (PRPL), which is to be submitted. Each section of this article corresponds to a chapter in PRPL, and vice versa. The proofs of all the results are in PRPL, so they are omitted in this article.