Information processing theory views thinking as the adaptation of activity to environmental inputs. A dual cognitive structure is proposed to explain this adaptation, in which thoughts are selected by interactions between an explicit memory system (accessible to consciousness) and an implicit memory system (inaccessible to consciousness). Contrary to commonsense assumptions about cognition, the implicit system plays the dominant role in skilled thinking. The explicit system has a largely regulatory function, overriding the implicit system when it encounters novel situations for which it has no response. Although the explicit system may be developed by cognitive process instruction in the classroom, the all-important implicit system is developed mainly by direct experience of the work environment independently of conscious information processing. Paradoxically, there are no grounds for declaring any part of medical science irrelevant to practical decision-making, as its effects on implicit memory are unknown. Nevertheless, it should be taught as the basis for regulating a process of thinking that is mainly implicit.