This paper is concerned with the relation between perceptual and cognitive factors in distance responses. It is suggested that the reduction in the magnitude of distance cues that occurs for far portions of the visual held results in the perceptual underestimation of these portions relative to nearer parts of the visual field. As O becomes aware of this error he will apply (often erroneously) a cognitive correction to perceived distance in an effort to achieve veridicality. In agreement with the size-distance invariance hypothesis (SDIH), the perceptual underestimation of far distances will result in the perceptual underestimation of the size of far objects. 'This in turn leads to the cognitive expectation that distant objects will appear small relative to near objects and conversely that objects which appear larger or smaller than normal must be closer or farther, respectively, than the distance at which they appear. 'This explanation is applied to the size-distance paradox, to the reported size and distance of familiar objects, to problems in testing the size-distance invariance hypothesis, and to changes in spatial responses as a function of age.