Though primarily a philosopher and mathematician, Descartes nevertheless had a profound influence on the development of psychology. Bom in La Hague, he attended a Jesuit school where he studied both theology and science. He went on to receive a law degree at the University of Poitiers, but did not practice this profession. Instead, he became a gentleman soldier, applying his scientific knowledge to military engineering, and later to problems of meteorology and glacier formation. During this period he had a prophetic dream which suggested that all science should be interrelated, and that physics can be reduced to geometry.After traveling extensively in Europe, Descartes returned to Paris to write, but found that his views conflicted with Church doctrine. For this reason he went to live in Holland, where he was assured of greater intellectual freedom than in France.Descartes was largely responsible for the mechanistic viewpoint in French psychological thought. However, he arrived at his mechanism through a circuitous route, since his view was based on a dualistic conception of substance. In his Discourse on Method he adopted a universal “methodical doubt” in an attempt to discover at least one indubitable idea on which to base all knowledge. This he found in the “Cog- ito, ergo sum (I think, therefore I am)” —that is, to him the only certainty was the existence of the self or soul as a thinking being. On this foundation he based his proof of the existence of God and his recognition of the existence of matter.Since the self was, by definition, an unextended, incorporeal mental substance (res cogitans), it contrasted sharply with the material or extended substance (res extensa) which geometry describes. This brought up two problems: first, since mind and body are essentially different, how does each of them operate? And second, since they are not only distinct but together, how do they interact?Descartes attempted to solve the first problem by showing that the rational acts of the soul and the mechanical activities of the body can be approached and studied in their own right. His account of the soul is closely allied with scholastic thought, for he viewed it as an indivisible, “unitary substance” which nevertheless contained innate ideas of God, geometrical maxims, etc. which could be accepted by reason because they were “clear and distinct.” He also held that the six elementary “passions de Tame” (passions of the soul)—wonder, love, hate, desire, joy, sadness—are rationally motivated. These philosophical doctrines had little direct effect on psychology, although the view of McDougall that behavior is governed by a set of innate tendencies or instincts, and the Gestalt emphasis on basic organizing principles of the mind, are sometimes traced back to Descartes’ general position.Descartes’ handling of “corporeal substance,” however, had a more direct influence on psychological investigations. Since mind and body were essentially different, he proposed that the body could be freely studied as a thoroughly mechanical phenomenon, an automaton—a view that received its impetus from the Galilean theories of mechanics, which were beginning to be accepted at the time. This theory helped to establish a milieu for the development of the scientific approach to man in general, and the field of physiological psychology in particular.Descartes himself made a number of contributions to the study of emotions, vision, the senses, and nerve conduction. Although he characterized emotions as passions of the soul, he nevertheless treated them almost as mechanical events, explaining them in terms of motions in the brain, blood, and vital organs. He described the functioning of the normal and diseased eye, and demonstrated that the crystalline body is a lens through which light waves form an image on the retina. He was aware that muscles operate in opposing pairs, and recognized that both nerve and muscle responses follow from stimulation of the sense organs. Though he pictured the nerves themselves as hollow tubes conducting “animal spirits” in either direction between the muscles and sense organs, he nevertheless offered a “pathway” theory of the peripheral nervous system, and came close to the modem concept of the reflex arc (Boring, 1950).Descartes took the position that the mechanical approach could be fully applied to animals, since they do «ot have a soul, but man is partly physical and partly spiritual in nature. This left him the problem of the relationship between the “substances.” He recognized that in spite of their essential differences, mind acts upon body and body upon mind, and proposed that the locus for this activity is in the tiny pineal body, naming the point of contact the “conarium” (which, according to Hinsie and Campbell, 1960, became Freud’s id). The reason for choosing this site was that the pineal body is located at the center of the brain, and was apparently the only part of the brain which was unitary and not duplicated. According to Descartes, its primary purpose is to direct the animal spirits to one or another part of the body, making possible mental activities like recognition and imagination, aswell as physical activities such as movement. Though crude, this theory introduced the idea of interaction, and when psychology eventually divorced itself from theology, it helped to give rise to a holistic approach to the human being.