I. _The anthropistic theory of consciousness._--It is peculiar to man. To Descartes we must trace the widespread notion that consciousness and thought are man's exclusive prerogative, and that he alone is blessed with an "immortal soul." This famous French philosopher and mathematician (educated in a Jesuit College) established a rigid partition between the psychic activity of man and that of the brute. In his opinion the human soul, a thinking, immaterial being, is completely separated from the body, which is extended and material. Yet it is united to the body at a certain point in the brain (the _glandula pinealis_) for the purpose of receiving impressions from the outer world and effecting muscular movements. The animals, not being endowed with thought, have no soul: they are mere automata, or cleverly constructed machines, whose sensations, presentations, and volitions are purely mechanical, and take place according to the ordinary laws of physics. Hence Descartes was a _dualist_ in human psychology, and a _monist_ in the psychology of the brute. This open contradiction in so clear and acute a thinker is very striking; in explaining it, it is not unnatural to suppose that he concealed his real opinion, and left the discovery of it to independent scholars. As a pupil of the Jesuits, Descartes had been taught to deny the truth in the face of his better insight; and perhaps he dreaded the power and the fires of the Church. Besides, his sceptical principle, that every sincere effort to attain the truth must start with a doubt of the traditional dogma had already drawn upon him fanatical accusations of scepticism and atheism. The great influence which Descartes had on subsequent philosophy was very remarkable, and entirely in harmony with his "book-keeping by double entry." The _materialists_ of the seventeenth and eighteenth centuries appealed to the Cartesian theory of the animal soul and its purely mechanical activity in support of their monistic psychology. The _spiritualists_, on the other hand, asserted that their dogma of the immortality of the soul and its independence of the body was firmly established by Descartes' theory of the human soul. This view is still prevalent in the camp of the theologians and dualistic metaphysicians. The scientific conception of nature, however, which has been built up in the nineteenth century, has, with the aid of empirical progress, in physiological and comparative psychology, completely falsified it.