The first way we can approach language is as a phenomenon of the individual person. It is concerned with describing and explaining language as a matter of human behavior. People speak and write; they also evidently read and understand what they hear. They are not born doing so; they have to acquire these skills. Not everybody seems to develop them to the same degree. People may suffer accidents or diseases, which impair their performance. Language is thus seen as part of human psychology, a particular sort of behavior, the behavior, which has as its principal, function that of communication. The trouble with the term “behavior” is that it is often taken to refer only to more or less overt, and describable, physical movements and acts. Yet part of language behavior-that of understanding spoken or written language, for example-has little or no physically observable signs. It is true we can sometimes infer that understanding has taken place by the changes that take place in the other person’s behavior. When someone has been prohibited from doing something, we may infer that he has understood the prohibition by observing that thereafter he never behaves in that way. We cannot, of course, be absolutely sure that his subsequent behavior is a result of his understanding; it might be due to a loss of interest or inclination. So behavior must be taken to include unobservable activity, often only to be inferred from other observable behavior. Once we admit that the study of language behavior involves describing and explaining the unobservable, the situation becomes much more complicated, because we have to postulate some set of processes, some internal mechanism , which operates when we speak and understand. We have to postulate something we can call a mind. The study of language from this point of view can then be seen as a study of the specific properties, processes and states of the mind whose outward manifestations are observable behavior; what we have to know in order to perform linguistically.This approach to language, as a phenomenon of the individual, is thus principally concerned with explaining how we acquire language, and its relation to general human cognitive systems, and with the psychological mechanisms underlying the comprehension and production of speech; much less with the problem of what language is for, that is, its function as communication, since this necessarily involves more than a single individual.