As suggested by its name, default logic [Reiter 1980] aims at providing a formal framework for default reasoning. To this end, default logic introduces some kind of nonmonotonic inference rules called defaults. Returning to Example 4.2.1, we know that, given any bird, a good policy is to believe that the bird is able to fly until a proof to the contrary, if any, can be established. Such common sense knowledge can be expressed by the default \(\frac{{BIRD(x):FLY(x)}}{{FLY(x)}}\) which is to be interpreted as “if x is a bird and if it is consistent that x can fly then infer that x can fly”.