 Applying the logic of extreme probabilities to real-world defeasible reasoning generates an obvious problem, however. We know perfectly well that, in the case of the default rules we actually use, the conditional probability of the conclusion on the premises is nowhere near 1. For example, the probability that an arbitrary bird can fly is certainly not infinitely close to 1. This problem resembles that of using idealizations in science, such as frictionless planes and ideal gases. It seems reasonable to think that, in deploying the machinery of defeasible logic, we indulge in the degree of make-believe necessary to make the formal models applicable. Nonetheless, this is clearly a problem warranting further attention.