René Descartes, who dedicated much time to the study of the pineal gland, called it the "principal seat of the soul."[28] He believed that it was the point of connection between the intellect and the body.[29] Descartes attached significance to the gland because he believed it to be the only section of the brain which existed as a single part, rather than one half of a pair. He argued that because a person can never have "more than one thought at a time," external stimuli must be united within the brain before being considered by the soul, and he considered the pineal gland to be situated in "the most suitable possible place for this purpose," located centrally in the brain and surrounded by branches of the carotid arteries.[28]

Baruch de Spinoza criticized Descartes' viewpoint for neither following from self-evident premises nor being "clearly and distinctly perceived" (Descartes having previously asserted that he could not draw conclusions of this sort), and questioned what Descartes meant by talking of "the union of the mind and the body."[30]

The notion of a "pineal-eye" is central to the philosophy of the French writer Georges Bataille, which is analyzed at length by literary scholar Denis Hollier in his study Against Architecture. In this work Hollier discusses how Bataille uses the concept of a "pineal-eye" as a reference to a blind-spot in Western rationality, and an organ of excess and delirium.[31] This conceptual device is explicit in his surrealist texts, The Jesuve and The Pineal Eye.[32]

The notion of an inner third eye (attributed mystical significance) also occurs in ancient, central and east Asian, and new age philosophies.