The important group of psychic activities which we embrace under the name of "emotion" plays a conspicuous part both in theoretical and practical psychology. From our point of view they have a peculiar importance from the fact that we clearly see in them the direct connection of cerebral functions with other physiological functions (the beat of the heart, sense-action, muscular movement, etc.); they, therefore, prove the unnatural and untenable character of the philosophy which would essentially dissociate psychology from physiology. All the external expressions of emotional life which we find in man are also present in the higher animals (especially in the anthropoid ape and the dog); however varied their development may be, they are all derived from the two elementary functions of the _psyche_, sensation and motion, and from their combination in reflex action and presentation. To the province of sensation, in a wide sense, we must attribute the feeling of _like_ and _dislike_ which determines the emotion; while the corresponding _desire_ and _aversion_ (love and hatred), the effort to attain what is liked and avoid what is disliked, belong to the category of movement. "Attraction" and "repulsion" seem to be the sources of _will_, that momentous element of the soul which determines the character of the individual. The _passions_, which play so important a part in the psychic life of man, are but intensifications of emotion. Romanes has recently shown that these also are common to man and the brute. Even at the lowest stage of organic life we find in all the protists those elementary feelings of like and dislike, revealing themselves in what are called their _tropisms_, in the striving after light and darkness, heat or cold, and in their different relations to positive and negative electricity. On the other hand, we find at the highest stage of psychic life, in civilized man, those finer shades of emotion, of delight and disgust, of love and hatred, which are the mainsprings of civilization and the inexhaustible sources of poetry. Yet a connecting chain of all conceivable gradations unites the most primitive elements of feeling in the psychoplasm of the unicellular protist with the highest forms of passion that rule in the ganglionic cells of the cortex of the human brain. That the latter are absolutely amenable to physical laws was proved long ago by the great Spinoza in his famous _Statics of Emotion_.