The time required to recognize that two perspective drawings portray objects of the same three-dimensional shape is found to be (i) a linearly increasing function of the angular difference in the portrayed orientations of the two objects and (ii) no shorter for differences corresponding simply to a rigid rotation of one of the two-dimensional drawings in its own picture plane than for differences corresponding to a rotation of the three-dimensional object in depth.

A psychological space is established for any set of stimuli by determining metric 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 (i) decays exponentially with this distance, and (ii) does so in accordance with one of two metrics, depending on the relation between the dimensions along which the stimuli vary. These empirical regularities are mathematically derivable from universal principles of natural kinds and probabilistic geometry that may, through evolutionary internalization, tend to govern the behaviors of all sentient organisms.

The present study explores some of the factors that determine how difficult a classification will be to learn or remember. By a "classification" we mean, here, simply a grouping of a given set of stimuli into two or more mutually exclusive and exhaustive classes. The learning or memorization of a classification can be regarded as a process of associating, to each stimulus, a certain response. This response might be the verbal label arbitrarily assigned to the class containing that stimulus, or it might be the act of sorting that stimulus into the bin arbitrarily assigned to its class. The essential feature of a classification task, however, is that the same response is assigned to several different stimuli. Accordingly, we reserve the term identification task for cases in which a different response is paired with each stimulus. In either case, the word "memorization" is intended, here, to refer to those con-

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