Discusses whether the sensation factors in visual depth perception have an original attribute of extension which is analogous but different to quality and intensity. Experiments on depth perception by Wundt, Hillebrand and Arrer are subjected to careful examination; and theories that make sensations of movement the essential factor of all space-perception have been outlined and criticized. Concludes that: (1) the third dimension is not a new content but is a form in which the contradictions between the two-dimensional attributes of the two groups of sensations (one from each eye) is eliminated (2) visual depth is conditioned by a relation of contradiction between the two-dimensional data of the retinal images or by the muscle-sensations connected with these images; and (3) depth is a form rather than a content of perception, and is not an original attribute of sensation. (PsycINFO Database Record (c) 2012 APA, all rights reserved)