Each person has his own way of experiencing the world. How a given situation is perceived and interpreted varies from one person to another, and may also vary from one occasion to another within the same person. Different psychological theories have tried to approach these basic facts in terms of various concepts. According to Piaget (1951), Neisser (1976) and others, each person has his own 'schemata", in terms of which he or she assimilates the world. According to George Kelly (1955), we all have our "personal constructs' which channel our psychological processes. Other psychologists speak in this connection of "mental structures' (e.g. Mandler 1975), 'semantic networks' (e.g. Anderson and Bower 1973), "memory structures' (Posner 1973), and "interassociated nodes in long-term memory" (Shiffrin and Schneider 1977).