This illustration from Ren矇 Descartes' Treatise of Man describes his conception of the mind and body as interacting but separate entities. The eyes in the figure are represented one above the other and focused on an arrow at right; lines extend from the eyes to the mid- and terminal points of the arrow. Descartes hypothesized that muscles change the lens of the eye in order to focus on objects at different distances. He believed that fluid from the pineal gland would automatically rush to the various muscles of the body, allowing them to move without involving the mind. Original woodcut by Gerard van Gutschoven (1664).