THIS subject, inasmuch as it is contained in the term learning theory, has become the central topic of the leading psychologists in the last generation or so. Beginning with an interest in animal psychology it has irradiated to become a means of constructing systems that are, in their way, as methodologically satisfactory as anything in the natural sciences. Logical constructs are built up and able to be put to the experimental test. The intention behind this line of psychological enquiry is to create a science of behaviour which is strictly experimental, and which will contain theories that can hold or fall with putting them to the test. One of the more convenient ways of doing this is to employ animals under laboratory conditions, as the fundamental processes of learning and behaviour are more easily seen in organisms that are lower down the evolutionary scale than man, and which are more amenable to experimental intereference. This does not necessarily mean that we can predict straightaway the behaviour of a man from the behaviour of a rat; rather it means that those mechanisms which are in man, complicated by the higher intellectual processes, are seen more clearly in the behaviour of the animal. Therefore a close and detailed investigation of the laws operating behind the animal's behaviour should help us to understand the basic human principles of behaviour.