Previous chapters took for granted animals’ ability to recognize when particular adaptive behaviors are appropriate. This chapter looks at some aspects of recognition and the organization of individual knowledge.

What does it mean to recognize something? In a formal sense, the answer is simple: It means to be in a unique state (as defined in Chapter 5) so that in the presence of object w, the animal is always in state W, and state W never occurs at any other time. This is also a necessary condition for the animal to discriminate w from things that are not w. But the formal answer is not much help in constructing machines or understanding organisms that are capable of recognition. Something more is required: We need to know how to process particular visual (or auditory, touch, or whatever) inputs; how to direct the visual apparatus on the basis of past information – where should the organism look next? How to distinguish objects from their backgrounds; how to identify the same object from different points of view. Above all, how to encode all this information so that it provides a useful basis for action.

Most of these questions are about perception, and I cannot do justice to them here. In order to get on with the study of learning and motivation, we must take for granted the processes that translate a particular physical environment into some internal state that allows the animal to recognize the environment on subsequent occasions. Perceptual processes are not trivial, but my main concern is with what the organism does. This chapter is about the last step in the process: the encoding of information in ways useful for action. I am concerned not with how the animal “sees” a Skinner box or a colored light, but with how these things resemble or differ from other things in his world.

It is not at all clear that the best way to answer this question is to begin with the concept of stimulus, where a stimulus is some very specific, physically defined event. Nevertheless, because the study of learning in animals grew up under the influence of reflex-oriented behaviorists, the relevant terms, experimental methods, and concepts have all evolved from “stimulus–response” psychology. Hence, “stimulus” is the natural place to start.