  In Ibn al-Haytham's account, if the size of an object  is known distance may be perceived through an inference; for a given size,  an object's distance is inversely proportional to its visual angle. Descartes recognized this traditional account, depending  as it does on past experience of an object's size  and on an inference or rapid judgment that combines perceived visual angle  with known or remembered size. Descartes held that these rapid judgments are habitual and happen so quickly that they go unnoticed. Further, the sensations that present the objects in accordance with visual angle also go unnoticed, as they are rapidly replaced by visual experiences of objects at a distance.