How many memory systems are there?

Memory is made up of a number of interrelated systems, organized structures of operating components consisting of neural substrates and their behavioral and cognitive correlates. A ternary clas- sificatory scheme of is proposed in which procedural, semantic, and episodic constitute a monohierarchical arrangement: Episodic is a specialized subsystem of semantic memory, and semantic is a specialized subsystem of procedural The three systems differ from one another in a number of ways, including the kind of consciousness that characterizes their operations. The ternary scheme overlaps with di- chotomies and trichotomies of proposed by others. Evidence for multiple systems is derived from many sources. Illustrative data are provided by ex- periments in which direct priming effects are found to be both functionally and stochastically independent of recognition Solving puzzles in science has much in common with solving puzzles for amusement, but the two differ in important respects. Consider, for instance, the jigsaw puzzle that scientific activity frequently imitates. The everyday version of the puzzle is determinate: It consists of a target picture and jigsaw pieces that, when properly assembled, are guaranteed to match the picture. Scientific puzzles are indeter- minate: The number of pieces required to complete a picture is unpredictable; a particular piece may fit many pictures or none; it may fit only one picture, but the picture itself may be unknown; or the hypothetical picture may be imagined, but its com- ponent pieces may remain undiscovered. This article is about a current puzzle in the science of It entails an imaginary picture and a search for pieces that fit it. The picture, or the hypothesis, depicts as consisting of a number of systems, each system serving somewhat different purposes and operating according to some- what different principles. Together they form the marvelous capacity that we call by the single name of memory, the capacity that permits organisms to benefit from their past experiences. Such a picture is at variance with conventional wisdom that holds to be essentially a single system, the idea that memory is memory. The article consists of three main sections. In the first, 1 present some pretheoretical reasons for hypothesizing the existence of multiple systems and briefly discuss the concept of system. In the second, I describe a ternary classifi- catory scheme of memory--consisting of procedural, semantic, and episodic memory--and briefly com- pare this scheme with those proposed by others. In the third, I discuss the nature and logic of evidence for multiple systems and describe some experiments that have yielded data revealing independent effects of one and the same act of learning, effects seemingly at variance with the idea of a single system. I answer the question posed in the title of the article in the short concluding section.