  Descartes realized that the central nervous state that controls accommodation would vary directly in proportion to the distance of objects. However, unlike the case of inferring distance from known size and visual angle, Descartes did not suppose that the mind is aware of the apparatus for controlling the accommodation of the eye. Rather, he supposed that, by an innate mechanism, the central brain state that varies with distance directly causes an idea of distance in the mind (6:137; 11:183). This physiologically produced idea of distance could then be combined with perceived visual angle in order to perceive an  object's size, as in al-Haytham's  theory of size perception. When we correctly perceive the  distance and combine it with visual angle (by an unnoticed mental act), the result is a veridical perception of a size-at-a-distance. Descartes described the resulting perception as possessing the attributes that were labelled as “size constancy”  in the twentieth century: