Fundamental to the differences among the several kinds of knowledge is the dichotomy between the concrete flow of experiences and the conceptual systems that are abstracted by thought from them. How individuals came to have knowledge was once a standard philosophical topic. Descartes posited certain innate ideas; Locke denied their existence and argued for tabula rasa on which ideas and impressions of sense were recorded, combined, and retrieved. Plato's theory of recollection and Aristotle's theory of cognitive functions are also accounts of how knowledge is acquired. As the empirical study of human behavior and mental functioning developed, the distinction became sharper between generalizations of the psychological processes and the structural-logical properties of knowledge. Different types of knowledge make different claims on the curriculum, and justifying their place in the curriculum very soon takes on philosophical dimensions.