Everybody recognises that the way people behave is in terms of how they know things to be. But where does this knowledge come from? How does it develop in the individual? These very general questions are a good beginning for understanding the need for a psychological input to the social sciences since learning, storage and use of this knowledge is both a mental and a social process.

There can only be three possible sources to the knowledge held by people. (1) It can come from an innate capacity, transmitted genetically from the parents, which either the child already possesses at birth, or which develops later, as he or she matures, much in the way that boys develop facial hair at adolescence. (2) It can come from the individual learning from the environment as she interacts with it. (3) It can come from learning from other individuals through some process of communication. For any part of knowledge we may be dealing with a combination of all three.