1. Introduction One of the fundamental questions of perceptual theory is whether the structured energy fields that are available to perceptual systems are sufficient, in and of themselves, for accurate perception. If potential sensory stimulation 1 is not sufficient, then accurate perception must depend upon op- erations carried out by the animal, such as inferential pro- cessing. Thus, the assumption that potential sensory stimu- lation is insufficient for accurate perception leads to the hypothesis that perception is indirect (i.e., accurate per- ception requires the addition, presumably mental, of infor- mation that is not available in sensory stimulation) which, in turn, leads theorists to focus on internal processing as the locus of the most important issues in perception. On the other hand, if potential sensory stimulation is sufficient for accurate perception, then perception can be direct, that is, accurate without the addition of information beyond what is available in sensory stimulation. The latter view is central to the ecological approach to perception and action (Gib- son 1979/1986). Proponents of the ecological approach stress that ambient arrays are structured by the animal-en- vironment interaction (that is, by the position and motion of the animal relative to its environment), and that this structuring is governed by physical law (i.e., laws of the propagation, reflection, and absorption of energy) in such a way that any given physical reality gives rise to a unique structure or pattern in ambient energy. This leads to the hy- pothesis that potential sensory stimulation is sufficient for accurate perception because the animal-environment in- teraction is specified in the spatiotemporal structure of am- bient arrays. Specification refers to a lawful, 1:1 relation be- tween patterns in ambient arrays and the aspects of the an- imal environment interaction that give rise to them (Shaw et al. 1982). The ecological approach to perception and ac- tion is an established theory with broad empirical support, and for this reason we do not review it at length here (for BEHAVIORAL AND BRAIN SCIENCES (2001) 24, 195–261 Printed in the United States of America © 2001 Cambridge University Press 0140-525X/00 $12.50 195 On specification and the senses Thomas A. Stoffregen Division of Kinesiology, University of Minnesota, Minneapolis, MN 55455 tas@umn.edu www.homepages.uc.edu/~stoffrta/psi/index.html Benoît G. Bardy Division of Sport Sciences (STAPS), Université Paris Sud-XI, 91405 Orsay Cedex France benoit.bardy@staps.u-psud.fr Abstract: In this target article we question the assumption that perception is divided into separate domains of vision, hearing, touch, taste, and smell. We review implications of this assumption for theories of perception and for our understanding of ambient energy ar- rays (e.g., the optic and acoustic arrays) that are available to perceptual systems. We analyze three hypotheses about relations between ambient arrays and physical reality: (1) that there is an ambiguous relation between ambient energy arrays and physical reality, (2) that there is a unique relation between individual energy arrays and physical reality, and (3) that there is a redundant but unambiguous rela- tion, within or across arrays, between energy arrays and physical reality. This is followed by a review of the physics of motion, focusing on the existence and status of referents for physical motion. Our review indicates that it is not possible, in principle, for there to be a unique relation between physical motion and the structure of individual energy arrays. We argue that physical motion relative to differ- ent referents is specified only in the global array, which consists of higher-order relations across different forms of energy. The existence of specificity in the global array is consistent with the idea of direct perception, and so poses a challenge to traditional, inference-based theories of perception and cognition. However, it also presents a challenge to much of the ecological approach to perception and action, which has accepted the assumption of separate senses. Keywords: epistemology; information; intersensory; perception; perceptual learning; sensory neurophysiology; sensory systems; speci- fication Thomas A. Stoffregen, Associate Professor in the Division of Kinesiology at the University of Minnesota, is the author of over 40 publications in the area of per- ception and action, including affordances, the percep- tion and control of spatial orientation, motion sickness, virtual reality, audition, and perceptual-motor develop- ment. He is a recipient of the Faculty Achievement Award from the University of Cincinnati, and his re- search has been funded by the National Science Foun- dation and the U.S. Navy. He is a Consulting Editor for Ecological Psychology, and serves on the Advisory Board for The Handbook of Virtual Environments Technology. Benoît G. Bardy, Professor of Sport Sciences, is the author of over 40 publications in the area of Human per- ception and action, including coordination dynamics, the perceptual regulation of posture and locomotion, and the control of spatial orientation. He is the director of the Center for Research in Sport Science at the Uni- versity of Paris Sud XI, and serves as an Associate Edi- tor for Ecological Psychology.