This article advances the thesis that the purpose of low-level vision is to encode symbolically all of the useful information contained in an intensity array, using a vocabulary of very low-level symbols: subsequent processes should have access only to this symbolic description. The reason is one of computational expediency: it allows the low-level processes to run almost autonomously: and it greatly simplifies the application of criteria to an image, whose representation in terms of conditions on the initial intensities, or on simple measurements made from them, is very cummbersome. The implications of this thesis for physiological and for computational approaches to vision are discussed. A list is given of several computational problems in low-level vision: some of these are dealt with in the accompanying articles.
