When applied to theories of animal learning and behavior, the term ‘‘cognitive” is usually defined by contrast or exclusion: A cognitive theory is one which rejects certain assumptions of the theoretical tradition which has long dominated the study of animal behavior, namely, S−R theory. In opening my chapter with this remark, I do not intend to set up a target for subsequent attack. My intention is rather to use the antithesis with S−R theory as a way of distinguishing some of the strands that go to make up cognitive theory. If I am correct in supposing that cognitive theories have been defined by contrast in this way, it is probable that there will be as many different cognitive theories as there are assumptions within S−R theory which someone may choose to challenge. It will be well to understand which one we are talking about on any given occasion.