According to the everyday understanding of the mind (called folk psychology), people's belief and desire causally combine to determine their intention, which in turn controls their action. However, recent empirical investigations have shown that psychologists' and laypeople's intuitions are not always in agreement about this common sense. To shed light, in this study folk psychology of belief, desire, and intention is conceptualized as a category of the mind; that is, a kind of category that embodies people's causal knowledge about human action. Four experiments explored the implications of this conception for understanding and prediction of social action (Experiments 1 and 2) and for explanation of simple scripted action (Experiments 3 and 4). The results showed that the present conception of folk psychology is useful in guiding empirical investigation. A systematic investigation of folk psychology may revitalize Heider's research program on naive psychology.Everyday understanding of human action requires an understanding of other minds. Ever since Premack and Woodruff (1978) posed the question, ''Does the chimpanzee have a theory of mind?'', humans' everyday understanding of the mind has become a major issue in psychological research. Often dubbed folk psychology, laypeople's conception of the mind has in recent times been investigated by philosophers of the mind (e.g., Churchland, 1979;Stich, 1983) and developmental psychologists (e.g., Leslie, 1987;Perner, 1991;Wellman, 1990). In the philosophy of the mind, arguments rage over whether folk psychology should be eliminated or retained in the scientific inquiry of the mind (for a recent collection of major viewpoints, see Greenwood, 1991). In developmental psychology, there is a sizable and still growing literature on children's acquisition of a theory of mind (see for a recent review, Gopnik, 1993;Siegal, 1996).Although there remain many points of dispute, the consensus among philosophers and developmental psychologists holds that the bedrock of a folk psychology consists in the notions of beliefs and desires (Stich, 1983;Wellman, 1990); hence, another nomenclature of belief-desire psychology. According to the accepted view, adults in at least Englishspeaking cultures hold that an actor performs an action if the actor wants an outcome and believes that the outcome can be obtained by performing the action. However, beliefs and desires do not immediately cause action, but rather intention is thought to mediate the causal