Abstract
A psychological space is established for any set of stimuli by determining metrix distances between the stimuli such that the probability that a response learned to any stimulus will generalize to any other is an invariant monotonic function of the distance between them. To a good approximation, this probability of generalization (1) decays exponentially with this distance, and (2) does so in accordance with one of two metrics, depending on the relation between the dimensions along which the stimuli vary.