The first really important book on the physiology of the passions was written by Descartes, the great restorer of science, who, with his prodigious force of intellect, embraced all branches of knowledge, and was at once mathematician, physicist, and physiologist. His is the honour of having shown that the old Aristotelian philosophy, then prevalent in the schools, had never solved one of the problems respecting life. In the treatise upon 'The Passions of the Soul,’ the following words appear in a section in which he investigates the manner in which the passions are excited: 'If the appearance of an animal is very strange and frightful--that is, if it has much resemblance with those things which were originally hurtful to the body, it will excite in the mind the passion of fear, then of boldness or of horror, according to the different temperaments of the body or the force of the soul, and according as one has been able or not to provide one’s self with the means of defence, or of flight from those dangerous things with which the present impression has points of resemblance. This in some men disposes the brain in such a manner that the spirits, excited by the image and formed in the pineal gland (or central part of the brain), pass thence, partly to the nerves which serve to turn the body and move the legs in flight, and partly to those nerves which enlarge and contract the valves of the heart, or stimulate the other parts, whence the blood is sent to them in such a manner that this blood, otherwise elaborated, sends spirits to the brain capable of fomenting and increasing the passion of fear; that is, they are able to keep open or reopen the pores of the brain which conduct them to the nerves.’[1] No one before Descartes had had so simple a conception of the mechanism by which the involuntary movements accompanying the emotions are produced, and he it is who laid the foundations of the physiological study of the mind. Two centuries and a half have already passed, and his work still remains a monument worthy of all admiration. Science has advanced so greatly that perhaps no one now who wished to learn the elements of physiology would study his treatise on man, and yet none who know the history of science but are moved by those marvellous pages out of which breathes that spirit of innovation which has fertilised the science of centuries. Malebranche relates that when he first took up the treatise--'L’homme et la formation du fœtus,’ by Descartes--the new ideas it stirred up within him gave him a pleasure so intense, and so filled him with admiration, that his heart palpitated and he was obliged to pause from time to time. [1] Œuvres de Descartes, _Les passions de l’âme_, xxxvi.