The term MEMORY arises when an environmental event at a given time results in an organism behaving differently at a later time. In everyday psychology the later behaviour is then attributed to the organism's having remembered the earlier event, and behaved accordingly. Or more commonly, when the later behaviour fails to occur it is said that the organism forgot the earlier event. Operant function-altering events (conditioning and extinction, the weakening of behaviour by punishment and the strengthening that results from recovery from punishment, the development of SD's, the developments of Sr's and Sp's, and others) change the organism in such a way that behaviour at a later time is different from what it would have been if the function-altering event had not occurred.