APA Dictionary of Psychology

a goal or intended result that directs mental processes. For example, when shown the numbers 6 and 4 and asked to add them, a person answers 10, but when asked to subtract them, they answer 2; the same stimuli lead to different thoughts as intended ends determine what the mind does with the two numbers. The term, introduced by German psychologist Narziss Ach (1871–1946) and derived from the earlier concept of Aufgabe, is essentially equivalent to the modern mental set.