缺省逻辑是Ray Reiter提出的用来形式化有缺省假定的推理的非单调逻辑。

缺省逻辑可以表达像“缺省的，某个事物是真的”的事实；相反的，标准逻辑只能表达某个事物为真或某个事物为假。这是一个问题，因为推理经常涉及在多数时候是真但不总是真的事实的推理。经典的例子是：“鸟通常会飞”。这个规则可以在标准逻辑中表达为要幺“所有鸟都会飞”，这与企鹅不会飞的事实相矛盾；要幺“除了企鹅、鸵鸟...的所有鸟都会飞”，这要求规则指定出所有的例外。缺省逻辑致力于形式化像这样的推理规则，而不需要明确提及所有的例外。

Default logic is a non-monotonic logic proposed by Raymond Reiter to formalize reasoning with default assumptions.

Default logic can express facts like “by default, something is true”; by contrast, standard logic can only express that something is true or that something is false. This is a problem because reasoning often involves facts that are true in the majority of cases but not always. A classical example is: “birds typically fly”. This rule can be expressed in standard logic either by “all birds fly”, which is inconsistent with the fact that penguins do not fly, or by “all birds that are not penguins and not ostriches and ... fly”, which requires all exceptions to the rule to be specified. Default logic aims at formalizing inference rules like this one without explicitly mentioning all their exceptions.