Introduction
external representation - information about objects and events, such information represents them and is ready for use by a perceptual system
internal representation - common and useful to all fields of cognitive science, not as important here
**Proper consideration of external representation is logically prior to internal representation and information processing; we must know what information is used before we can understand how it is processed
*The study of perceptual information needs a methodology.
**Information for a perceiver is not in frequency, intensity, mass, extent, or time; it is distributed across them.
**Although distortions can be found in static cross sections of the optic array, they seem less noticeable (or less disruptive) in dynamic images. There, movement of the observer, motions of an object, or both reveal three-dimensional relations. Projective geometry is the body of rules for that revelation, and the perceptual system appears to know some of them.
**For every invariant that may exist, perception gets easier to understand and to implement on a machine.