The goals of the computation that extracts disparity from pairs of pictures of a scene are defined, and the contraints imposed upon that computation by the three-dimensional structure of the world are determined. Expressing the computation as a grey-level correlation is shown to be inadequate. A precise expression of the goals of the computation is possible in a low-level symbolic visual processor: the constraints translate in this environment to pre-requisites on the binding of disparity values to low-level symbols. The outine of a method based on this is given.