The secretory activity of the pineal gland is only partially understood. Its location deep in the brain suggested to philosophers throughout history that it possesses particular importance. This combination led to its being regarded as a "mystery" gland with mystical, metaphysical, and occult theories surrounding its perceived functions.
Ren? Descartes, dedicating much time to the study of the pineal (Pine-cone-shaped) gland, has called it the "principal seat of the soul."[34] He believed it to be the point of connection between the intellect and the body.[35] Descartes attached significance to the gland because he believed it to be the only section of the brain to exist 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.[34]
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."[36]
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.[37] This conceptual device is explicit in his surrealist texts, The Jesuve and The Pineal Eye.[38]
Numerous spiritual philosophies contain the notion of an inner Third Eye that is related to the ajna chakra and also the pineal gland, and to which is attributed significance in mystical awakening or enlightenment, clairvoyant perception, and higher states of consciousness. This idea occurs historically in ancient, central, and east Asia; and also in contemporary occult theories relating to yoga, Theosophy, Pagan religions, and New Age spiritual philosophies.