 So it is possible to learn, by trial and error, with error-correction, to detect and define what distinguishes dogs from non-dogs, and hence to correctly categorize them.This kind of learning, called reinforcement learning in the behavioral literature and supervised learning in the computational literature, is fundamentally dependent on the possibility of error, and error-correction.Miscategorization -- examples of nonmembers of the category -- must always exist, not only to make the category learnable, but for the category to exist and be definable at all.