This paper argues that an ecological approach to psychology of the sort advanced by J. J. Gibson provides a coherent and powerful alternative to the computational, information-processing, paradigm. The paper argues for two principles. Firstly, one cannot begin to understand what internal information processing an organism must accomplish until one understands what information is available to the organism in its environment. Secondly, an organism can process information by acting on or manipulating physical structures in its environment. An attempt is made (...) to show how these principles can be extended to cognition as a whole. It is suggested that these principles may have a foundation in evolutionary biology. (shrink)