What is the relation of visual perception to the measured order of physical objects? Both vision and measurements are sources of information about physical objects, although they provide this information in different ways and with systematic differences of organization. Although vision is our primary source of information about the external world, it does not supply the whole truth unassisted. Perceptual data do not arrange themselves without ambiguity, nor are they transformed automatically into all the information we desire. The various uses we make of visual data shape the form of knowledge, and to a surprising degree they also shape its content.