We have here to do with a question of genesis; we must therefore begin with the humblest phenomena. The primordial sense, the only one in certain animals, is touch combined with internal sensations. Let us remark that, in its origin, the “knowledge” which we take in its lowest degree has only a _practical_ value; sensation is a monitor, an aid, an instrument, a weapon with only one aim—the preservation of the individual,—and completely subordinated to that end; otherwise, it is nothing but a useless manifestation, a luxury. The _nexus_ between the sensations and the organic and motor reactions is therefore innate—_i.e._, it results from the very constitution of the animal. If it fails, the conditions of existence are at fault. The primordial tissue, says Spencer, must be differently affected, according as it is in contact with nutritive matter (ordinarily soluble) or with innutritive matter (ordinarily insoluble). The contraction by which the tactual surface of a rhizopod absorbs a fragment of assimilable matter is caused by a commencing absorption of this matter, _i.e._, contact and absorption are the same thing. The action of certain agents is followed by a retractile movement, or, on the contrary, by movements of a character to assure the continuance of the impression. These two kinds of movement are, in this writer’s view, respectively the phenomena and the signs of pleasure and pain. The tissue, therefore, acts in such a manner as to assure pleasure and avoid pain, by a law as physical and natural as that by which a magnet turns towards the pole, or a tree to the light. Without inquiring whether pleasure and pain exist in this case—a purely hypothetical assumption—there are, at least, objective phenomena denoting a _nexus_ of utility between the sensation and the expansive or retractile movements. Passing from these inferior organisms to those provided with several senses, we find no change. Each order of sensation acts in the same way. The animal is better informed, and consequently better protected and armed—that is all. Finally, when certain images (_i.e._, recollections of pleasures and pains experienced) excite an emotional state, the mechanism remains the same, and tends towards the same end. It is therefore not without reason that we have above assimilated every form of primary emotion to a psycho-physiological organism adapted to a particular end. It is needless to review the primary emotions and to show that the sensation, the perception, or the image only produces organic or motor troubles when the preservation of the individual or the species is at stake. The intellectual state (sensation, perception, or image) can instinctively—_i.e._, through an innate mechanism—produce immobility, oppression, withdrawing into one’s self, flight (fear); or, on the contrary, aggressive movements, attack (anger), or movements of attraction, accompanied by phenomena peculiar to each species (sexual love).