Innate Ideas Revisited

The notion of innate ideas has long been the subject of intense debate in the fields of philosophy and cognitive science. Over the past few decades, methodological advances have made it possible for developmental researchers to begin to examine what innate ideas—what innate concepts and principles—might contribute to infants’ knowledge acquisition in various core domains. This article focuses on the domain of physical reasoning and on Spelke’s (1988, 1994) proposal that principles of continuity and cohesion guide infants’ interpretation of physical events. The article reviews recent evidence that these two principles are in fact corollaries of a single and more powerful principle of persistence, which states that objects persist, as they are, in time and space. Is an infant’s knowledge about the physical world derived solely from the application of domain-general processes to experience? Or does it also reflect the contribution of innate ideas—concepts and principles specific to the domain of physical reasoning that guide an infant’s interpretation of physical events from birth? The notion of innate ideas, first introduced by Plato, was developed extensively in the 17th century by rationalist philosophers such as René Descartes and Gottfried Wilhelm Leibniz. They proposed that some ideas are part of our rational nature and that although experience may be necessary to bring them to consciousness, it does not determine their form. These proposals were criticized by empiricist philosophers such as John Locke in the 17th century, David Hume in the 18th century, and John Stuart Mill in the 19th century. The empiricist thesis held that innate ideas were superfluous, because knowledge acquisition could be explained more parsimoniously in terms of the application of domain-general (and often species-general) processes to experience. Empiricist approaches prevailed in philosophy and psychology well into the 20th century: Consider, for example, the behaviorism of John B. Watson (1924) and B.F. Skinner (1938) or the constructivism of Jean Piaget (1954). The notion of innate ideas was finally revived in the mid-20th century when the linguist Noam Chomsky (1965) proposed that human infants are born with a universal grammar that makes possible their rapid acquisition of language. Chomsky’s theory departs from earlier rationalist proposals in at least two significant ways. First, the universal grammar is understood to be an unconscious language-acquisition system, rather than a set of ideas that can be brought to consciousness by appropriate triggers. Second, the system is construed as a biological adaptation whose existence is rooted in the process of evolution, rather than in metaphysics (for reviews, see Chomsky, 1965; Markie, 2004; Pinker, 2003; Samet, 1999). In the following decades, Chomsky’s (1965) views were adopted by many (though by no means all) cognitive scientists. In the field of developmental psychology, methodological advances made it possible to begin to explore experimentally what innate concepts and principles might contribute to infants’ knowledge acquisition in various domains, including physical reasoning, psychological reasoning, and number (e.g., Gergely, Nádasdy, Csibra, & Bı́ró, 1995; Leslie, 1987, 1994; Premack & Premack, 1995; Spelke, 1988; Wynn, 1992). Within this body of developmental work, Elizabeth Spelke’s proposal, that principles of continuity and cohesion guide infants’ interpretation of physical events, has been highly influential (e.g., Spelke, 1988, 1994; Spelke, Breinlinger, Macomber, & Jacobson, 1992; Spelke, Phillips, & Woodward, 1995). The principle of continuity states that objects exist and move continuously in time and space: They cannot spontaneously appear or disappear (continuity), nor can they occupy the same space as other objects (solidity). The principle of cohesion states that objects are connected and bounded entities: They cannot spontaneously fragment as they move (cohesion) or fuse with other objects (boundedness). Although this point is often misunderstood by empiricist researchers, claims about innate ideas are of course empirical, and as such they are subject to revision in light of new experimental findings. In this article, I review evidence suggesting that the principles of continuity and cohesion represent only two corolAddress correspondence to Renée Baillargeon, Department of Psychology, University of Illinois, 603 East Daniel Street, Champaign, IL 61820; e-mail: [email protected]. PERSPECTIVES ON PSYCHOLOGICAL SCIENCE 2 Volume 3—Number 1 Copyright r 2008 Association for Psychological Science laries of a single and more powerful principle of persistence, which states that objects persist, as they are, in time and space. CONTINUITY, COHESION, AND PERSISTENCE Initial investigations of infants’ physical reasoning revealed three main findings relevant to Spelke’s proposal that infants interpret physical events in accordance with the principles of continuity and cohesion. First, infants succeeded in detecting several different continuity violations: They were surprised when shown events in which objects magically disappeared or occupied space already occupied by other objects (e.g., Baillargeon, Spelke, & Wasserman, 1985; Spelke et al., 1992). Second, infants detected cohesion violations when shown events in which objects broke apart as they moved (e.g., Needham, 1999; Spelke, Breinlinger, Jacobson, & Phillips, 1993). Third, infants failed to detect several different change violations when shown events in which objects surreptitiously changed size, shape, pattern, or color (e.g., Newcombe, Huttenlocher, & Learmonth, 1999; Simon, Hespos, & Rochat, 1995). These findings led to the commonly held view that young infants expect objects to exist continuously and to remain cohesive, in accordance with the principles of continuity and cohesion, but that they have no expectation about objects’ individual properties until they learn for each object category, which spontaneous changes are possible and which are not (e.g., Scholl & Leslie, 1999; Xu & Carey, 1996). In this view, infants’ physical world is thus, in part, a fairy-tale one: Although cups can neither magically disappear nor break apart, they can spontaneously change into pumpkins. Subsequent investigations cast doubt on this characterization of the infant’s physical world: They revealed that infants could detect some continuity violations but not others and some change violations but not others (e.g., Hespos & Baillargeon, 2001a; Wilcox, 1999). To make sense of these conflicting findings—and to sort out the conditions in which infants do and do not detect continuity and change violations—my collaborators and I developed a new account of infants’ physical reasoning (Baillargeon, Li, Luo, & Wang, 2006; Baillargeon, Li, Ng, & Yuan, in press). Our account assumes that infants’ representations of events— or physical representations—are initially impoverished but become richer with experience as infants gradually learn what information to include in order to better predict outcomes. Any information infants include in their physical representations becomes subject to a principle of persistence, which incorporates and extends the principles of continuity and cohesion. The persistence principle states that objects not only exist continuously and remain cohesive, they also retain their individual properties. According to this principle, no object can undergo a spontaneous or uncaused change in the course of an event, be it winking out of existence; breaking apart; or changing size, shape, pattern, or color. Of course, outside of the laboratory, objects rarely undergo such spontaneous changes, so an expectation of persistence is highly adaptive. According to our account, infants succeed in detecting continuity and change violations when they have included the necessary information to do so in their physical representations. Thus, infants cannot be surprised when a wide object is lowered inside a narrow container (a continuity violation) or when a narrow object is much wider after being briefly lowered inside a wide container (a change violation) if they have included no width information in their physical representation of the event. Consistent with this analysis, infants who fail to detect a continuity or a change violation in an event, because they have not yet learned to include the necessary information in their physical representation of the event, succeed in detecting the violation if induced (through contextual manipulations) to represent the information. Once included in the physical representation, the information becomes subject to the persistence principle, and the event is flagged as a violation. The preceding account suggests that the physical world of infants is not, in fact, a fairy-tale one. If infants represent objects as small and cuplike (either on their own or as a result of contextual manipulations), they expect them not to change spontaneously into objects that are large and pumpkinlike. In the following sections of this article, I explain in more detail how infants’ physical representations develop. I then return to the claims made in this section and review some of the evidence that supports them. AN ACCOUNT OF INFANTS’ PHYSICAL REASONING Our account of infants’ physical reasoning (Baillargeon et al., 2006, in press) assumes that when infants watch a physical event, their physical-reasoning system—an abstract computational system designed to monitor events as they unfold and to interpret and predict their outcomes—builds a specialized physical representation of the event. Any information included in this representation is interpreted in terms of infants’ core concepts and principles. Infants are said to be surprised in violation-of-expectation tasks when they look longer at events that violate, as opposed to confirm, their expectations; the term surprised is thus used here simply as a shorthand descriptor to denote a state of heightened attention or interest induced by an expectation violation. For many years, researchers expressed concerns over the interpretation of violationof-expectation findings, in part because of the gap between these findings and those of action tasks assumed to tap the same physical knowledge. Fortunately, these concerns have begun to dissipate as more sensitive action tasks have confirmed findings from violation-of-expectation tasks (e.g., Goubet & Clifton, 1998; Hespos & Baillargeon, 2006, in press; Hofstadter & Reznick, 1996; Hood & Willatts, 1986; Kochukhova & Gredeback, 2007; Li & Baillargeon, 2007; Ruffman, Slade, & Redman, 2005; von Hofsten, Kochukhova, & Rosander, 2007; Wang & Kohne, in press). If we define change violations as violations in which the properties of objects undergo spontaneous changes, then technically cohesion violations are also change violations. However, for clarity’s sake, in this article I follow tradition and distinguish between cohesion and other change violations. Volume 3—Number 1 3 Renée Baillargeon