In the Traité des passions [Art. 6] Descartes says that the body of a living man differs from that of a dead man as a watch or other automaton wound up and running according to the principle of its movement differs from a machine when it is broken and the principle of its movement ceases to operate. The struggle, he says [Art. 47], that people imagine between the higher and reasonable, and the lower and appetitive, parts of the soul, is in reality nothing but a disturbance in the pineal gland that occurs when the esprits animaux push the gland from one side while the soul through the agency of the will [which, in this case, resists the passion caused by the movement of the esprits] pushes it from the other side. In defining the passions of the soul [Art. 27] as "des perception, ou des sentiments, ou des émotions de l'â me, qu'on rapporte particulièrement à elle" [as opposed to other "sentiments" like odors, sounds, and colors, that one refers to exterior objects, or like hunger, thirst, and pain that one refers to the body], Descartes declares that they are caused, maintained, and strengthened by the movement of the esprits. These he defines as "un certain air ou vent très subtil" [Art. 7] produced in the brain by a complex action of the circulation [Art. 10]. Set in motion by perception or by the imagination the esprits move about the body via the nerves, those "petits tuyaux qui viennent tous du cerveau" [Art. 7], causing the passions of admiration, love, hate, desire, joy, and sadness and bodily movements and facial expressions that accompany them. Throughout the Traité des passions, the passions and their external manifestations are treated as physical reflexes, the inevitable and immediate result of changes in the machine of the body; and it is this mechanistic theory of matter, or "extension" as Descartes calls it, applied to the microcosm of the human body that Le Brun took over when he composed his own treatise on the passions. But Descartes, although he believed that "extension" functioned according to its own mechanical laws, and that no action of the reason or will could prevent experience or recurrence of the passions, believed nevertheless that they could be controlled, and that the man of virtuous life whose conscience never reproached him with having failed to do those things that he judged to be the best would have complete protection against the most violent efforts of the passions to disturb the tranquillity of his soul [Art. 148]. Furthermore Descartes did not, like the Stoics of antiquity, consider the passions as morbid states of the soul. As a Neo-Stoic of the Baroque age, sharing its fervent interest in the investigation of the physical universe, he considered them "toutes bonnes de leur nature" [Art. 211], believing that they needed only to be controlled; and if those men who were most moved by them experienced the greatest bitterness in life, so did they also taste the greatest sweetness. The soul could have its pleasures apart. But those which it shared with the body depended entirely on the passions [Art. 212].
le Brun who does not, like the philosopher, view the mechanistic theory of the passions in any large philosophical perspective, sums it up in the following passage wherein, after stating that ordinarily all that causes passion in the soul causes action in the body [by action he means any movement, bodily or facial], he traces this action back to its source in the circulation of the blood which generates the esprits:
"L'action n'est autre chose que le mouvement de quelque parti, et le changement ne se fait que par le changement des muscles, les muscles n'ont de mouvement que par l'extrémité des nerfs qui passent au travers, les nerfs n'agissent que par les esprits qui sont contenus par les cavités du cerveau, et le cerveau ne reçoit les esprits que du sang qui passe continuellement par le cìur, qui l'échaufe et le rarefie de telle sorte qu'il produit un certain air subtil qui se porte au cerveau, et qui le remplit" [Traité des passions in Jouin, Charles Le Brun, p. 372].
For the debt of the Cartesian physiology to medieval science and to Harvey's theory of the circulation of the blood, see E. Gilson, Études sur le rôle de la pensée médiévale dans la formation du systême cartésien, Paris, 1930, pp. 51-100. [p. 73]
[Lee, Rensselaer W. Ut Pictura Poesis, The Humanistic Theory of Painting. New York: W. W. Norton & Co., Inc. 1967.]