Defeasible reasoning

Defeasible reasoning is the study of forms of reasoning that, while convincing, are not as formal and rigorous as deductive reasoning. It has been discussed in philosophy and, more recently, in artificial intelligence.

Other alternatives to deductive reasoning include inductive reasoning and retroductive reasoning. These are not traditionally covered by most uses of the term "defeasible reasoning".

Origins in philosophy
Though Aristotle differentiated the forms of reasoning that are valid for logic and philosophy from the more general ones that are used in everyday life (see dialectics and rhetoric), subsequent philosophers mainly concentrated on deductive reasoning.

It wasn't until logical positivism started falling out of favour that philosophers like Roderick Chisholm and John L. Pollock renewed an interest in defeasible reasoning.

Artificial intelligence
Around the same time period, developments in artificial intelligence led pioneers like John McCarthy and Patrick J. Hayes to represent a form of defeasible reasoning as they encountered the frame problem and the qualification problem.

Several forms of defeasible reasoning were proposed:
 * McCarthy suggested that the solution was in a logical principle of circumscription
 * Raymond Reiter proposed a system of default logic and a formalization of the closed world assumption
 * Drew McDermott and Jon Doyle proposed non-monotonic logic
 * Robert C. Moore proposed autoepistemic logic
 * Donald Nute proposed defeasible logic