The scientific work of the seventeenth century was brought to its highest level by Galileo and Newton. The brilliance of these men set a fashion which extended to all branches of science and created the belief that the science of organisms could be reduced to the terms of a mechanical explanation. Descartes led the way by making a sharp distinction between mind and matter, with the intention of treating the organic activities by the method of the physical sciences. On this basis he was able to describe bodily movements as due to impressions on the sense-organ which set in motion the animal spirits : when this motion arrives at the brain it causes the opening of a valve through which the current then flows down the motor nerve. To this total motion, inward and outward, Descartes gives the name undulatio reflexa. As at that time the animal spirits were supposed to perform the opera tion of conduction along the nerves, this theory may be recog nized as a laudable attempt to state the principles of reflex action, not unlike the modern scheme of afferent and efferent currents with connection at a synapse. Descartes grasped very dearly the possibility of purely mechanical action, as it is actually seen in reflex movement. He decided that animals act always in this manner and stated the view that animals are machines. As he admitted a soul in human beings, they were not described as ma chines : the soul could direct the movement of the animal spirits by its action on the pineal gland in the brain, and this action constituted voluntary motion as superior to reflex action. The general idea of animal spirits and their motions was also used to explain emotions. Descartes saw the difficulty of explaining emo tions as consisting only in perceptions or ideas. He grasped the point that in emotions the peculiar element is the bodily disturb ance, and that the state of emotion is due to the combination of a perception with some tendency to motion, e.g., flight in the case of fear. This theory was further elaborated by Malebranche who adopted the Cartesian doctrine and through Malebranche it was handed on finally to modern times when it was more ade quately stated by Lange (1887). What is called the James Lange theory of the emotions is the modern version of the views expressed by Malebranche. In the eighteenth century La Mettrie drew the further conclusion that man is a machine. His work, L'Homme Machine (1748), was resented at the time as atheistic, but in fact it was little more than a conscientious attempt to con struct a complete physiological psychology.
Meanwhile in England the Cartesian principles inspired John Locke to attempt a fresh analysis of the mental powers. Locke's Essay concerning the Human Understanding (169o) was not strictly a work on psychology but it suggested a method of analys ing experience which led others to adopt views that were more distinctly psychological. Locke himself was interested in the analysis of knowledge and proposed to show that all human knowl edge is acquired during life by the use of natural faculties. Dis pensing with innate ideas and obscure "powers" of the soul, Locke employed only sensation and reflection. Sensation accounts for knowledge of the outer world : reflection enables us to perceive the inner events and attain ideas of such objects as memory. The problem which Locke created by this analysis, which is only partly genetic and never purely psychological, was that of finding a principle of unity which would hold together the separate ideas got by experience. The soul was no longer invoked as the prin ciple of unity. In its place was put the process of association. Locke barely mentions this and only recognized it as a form of connection when no rational union between ideas was evident. But his successors created what is known historically as the doc trine of association of ideas. David Hume (1711-1776) reduced experience to its lowest terms, impressions and ideas. He revived
Aristotle's doctrine of connection by similarity and contiguity, adding for reasons connected with his own philosophy the form of connection called cause and effect. In this way a completely em pirical plan was evolved. Nothing was required except the phenom ena of consciousness and the bonds between them established by habitual coexistence or succession. In the question of method Hume established the guiding principles of modern psychology. From Hume through James Mill to Bain, this standpoint was re tained as a practical method of psychological investigation. Its great merit was the emphasis laid on primary data and actual experience, with no metaphysical principles to confuse the issues. Its defects could only be seen when biological principles showed its limitations.
The psychologists of the eighteenth century in England were dominated by their ideals of scientific method. To make the mind intelligible they were ready to break it into atomic parts and invent ways of putting it together again. From England this idea spread to France. Already in France the spirit of Descartes had developed into a rationalism which tolerated nothing but the most sharply defined objects. The age of La Rochefoucauld's epigrams, La Bruyere's characters and all the wit of the salon, was a period when the psychological mood triumphed. The last phase was presented by ttienne Bonnot, Abbe de Condillac, chiefly in his Traite des Sensations (1754). Condillac's way of using the analytic method was to imagine a statue and endow it with one sense only, the sense of smell. In this way Locke's method is simplified : for while Locke began with sensation and reflection, Condillac starts with an isolated sensation and gradually builds up the mind by adding other sensations, the functions of attention and memory emerging in the process. If this procedure had the virtue of avoiding all mysticism and requiring no such hypothesis as a soul, it demonstrated all the vices that ruin the attempt to build a psychology out of logical abstractions. The successors of Condillac, the Ideologists of the Napoleonic period, were con scious of the failure but unable to remedy the situation. For a time psychology in France ceased to be important.
The German historians date the beginning of their historical achievements in psychology from the work of Leibniz who died in 1716. Though Leibniz wrote no separate work on psychology he made a number of suggestions which gradually acquired impor tance as the basis of a reaction against the tradition derived from Locke. Leibniz struck at the root of Locke's influence by insist ing that the activity of the intellect must be assumed from the first. This doctrine was characteristic of the German way of thinking and was partly due to the religious mysticism which persisted in that country from the 14th century onwards. Leibniz united his faith in the original activity of the intellect with the idea that all perceptible quantities can be divided into infinitesi mal parts. Assisted by the new ideas of the calculus in mathe matics, Leibniz evolved the theory that perception and will could be traced back to elementary forms which were too undeveloped to be presented in consciousness. He argued that the sound pro duced by a wave of the sea must be compounded out of the sounds made by each drop, and therefore that a perceptible sound would be the sum of a number of imperceptible causes. These were called "petites perceptions" and the level at which they enter consciousness was called apperception. Similarly a fully conscious act of will is the complete form of minute unnoticed tendencies to action. In this way Leibniz opposed the view that mental pro cesses must either be clearly apprehended or not exist at all. He asserted the reality of obscure and confused states of mind, and thus introduced views which were to be developed later into the theory of the threshold and the Unconscious.