Abstract Plausible reasoning concerns situations that have an inherent lack of precision, which 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. Several important plausible-reasoning examples that guide the development of some of the principles are also given. Versions of these principles and examples have appeared in Billington (2017) and chapter 4 of Billington (2019). Each principle in this article is the same as, or an improvement on, the corresponding principle in chapter 4 of Billington (2019); for a detailed comparison, see the Appendix. A propositional plausible logic that satisfies all these principles appears in Billington (2017, 2019). This shows that all the principles together are consistent.