Defeasible reasoning

In logic, defeasible reasoning is a kind of reasoning that is rationally compelling though not deductively valid. The distinction between defeasibility and indefeasibility may be seen in the context of this joke:

The engineer in this story has reasoned defeasibly; since engineering is a highly practical discipline, it is receptive to generalizations. In particular, engineers cannot and need not defer decisions until they have acquired perfect and complete knowledge. But mathematical reasoning, having different goals, inclines one to account for even the rare and special cases, and thus typically leads to a stance that is indefeasible.

Defeasible reasoning is a particular kind of non-demonstrative reasoning, where the reasoning does not produce a full, complete, or final demonstration of a claim, i.e., where fallibility and corrigibility of a conclusion are acknowledged. In other words defeasible reasoning produces a statement or claim. Other kinds of non-demonstrative reasoning are probabilistic reasoning, inductive reasoning, statistical reasoning, abductive reasoning, and paraconsistent reasoning. Defeasible reasoning is also a kind of ampliative reasoning because its conclusions reach beyond the pure meanings of the premises.

The differences between these kinds of reasoning correspond to differences about the conditional that each kind of reasoning uses, and on what premise (or on what authority) the conditional is adopted:

Defeasible reasoning finds its fullest expression in jurisprudence, ethics and moral philosophy, epistemology, pragmatics and conversational conventions in linguistics, constructivist decision theories, and in knowledge representation and planning in artificial intelligence. It is also closely identified with prima facie (presumptive) reasoning (i.e., reasoning on the "face" of evidence), and ceteris paribus (default) reasoning (i.e., reasoning, all things "being equal").

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