Abstract
This paper reviews theoretical developments in the study of discrimination learning the relation to 2 major contemporary viewpoints-a perceptual orientation as expressed in differentiation theory and a learning orientation as expressed in mediation theory. Early as well as present theory and research are found to have consist emphasized response processes, particularly those of a mediation nature, kin accounts of discrimination learning. This in of development is seen as tending to have precluded proper consideration of the nature and role of the stimulus. When viewed in this historical context, the conceptions of differentiation theory are seen as promoting a desirable theoretical and empirical balance.