ABSTRACT
This article, which I dedicate to the memory of James J. Gibson, is an expanded version of the Gibson Memorial Lecture, which I gave at Cornell University on October 21, 1983. I thank the members of the Department of Psychology at Cornell for providing me with an opportunity to clarifY the relation of my thinking to Gibson's, and the National Science Foundation for supporting both the preparation of this article and most of the research on which it is based (especially through Grants GB31971X, BNS 75-02806, and BNS 80-05517).
