but not to the systematic psychologist. Even the results of Ebbinghaus and of G. E. Miiller were discussed more often at the level of the educational psychologist than at that of the general psychologist. Slowly the perspective has been changing until now the concepts of learning are, to a large number of psychologists, among the most pervasive of the concepts of psychology. 2 We shall begin this review with a discussion of general problems.Constancy Attitude Versus Relativity Attitude. In a paper of the first importance for both theory and experiment Carr (30) has contrasted two attitudes toward experimental problems. The older and still the more common is the constancy attitude which assumes that psychological phenomena are constants and that the variations which measurements show are the outcome of masking conditions which conceal or distort the " true " values. Experimenters dominated by this attitude search for single or absolute curves of learning and retention, for " true " memory spans and for many other such " true" phenomena. Opposed to this is the relativity attitude which assumes that any given phenomenon is a function of N variables and seeks to find what these variables are and to measure their influence. A given phenomenon may be affected by some variable conditions and not by others, but only when we adopt the relativity attitude can we expect to work out adequate scientific descriptions 3 in which both variables and constants appear in a measured perspective. That the relativity attitude is becoming more prevalent in work on learning is, to the reviewer, one mark of the scientific coming-of-age of the psychology of learning.Descriptive Formulations of Learning. There have been, during the four-year period, a number of restatements of what is meant by learning and of what its problems are. Thorndike, in The Fundamentals of Learning and in a series of papers, has defended a " new connectionism," less rigid than the old and extended by such new concepts as belongingness, identifiability and system until the older 2 Klein (110), for example, cites learning as the fundamental criterion of differentiation between psychology and physiology.3 Cf. Dodge's (54) suggestion that a newer psychophysiological generalization, " Without variability, no mind," should be substituted for the old dictum, " Without phosphorus, no thought." He 'adds: " But the variations must apparently be of definite and distinctive kinds, connected in a specific manner with systematizations of relative persistency." Cf., also, H. M. Johnson's " Some Follies of Emancipated Psychology, Psychol. Rev., 1932, 39, 293-323, in which he points out certain cases of generalization in learning without regard to the restrictive conditions.