An illustration showing a theory of vision from Rene Descartes DE HOMINE (Treatise on Man) 1677. A human profile, with the eyes represented above each other and focused on an arrow at right. Lines extend from the eyes to the mid and terminal points of the arrow. Within the head, lines connect the optic nerves to the pineal gland, which connects to nerves leading to the arm which points at the arrow, establishing a relationship between the sensory perception of the arrow and muscular action in response.