 Philosophers and theorists of artificial intelligence have found a wide variety of applications for defeasible reasoning. In some cases, the defeasibility seems to be grounded in some aspect of the subject or the context of communication, and in other cases in facts about the objective world. The first includes defeasible rules as communicative or representational conventions and autoepistemic (reasoning about one’s own knowledge and lack of knowledge). The latter, the objective sources of defeasibility, include defeasible obligations, defeasible laws of nature, induction, abduction, and Ockham’s razor (the presumption that the world is as uncomplicated as possible).