A being, whatever it may be, whether vague and almost amorphous or clearly defined, is not isolated in the universal vital environment. It is the molecule of a diapason. It vibrates, not of its own accord, but in obedience to a general movement. The living cell, itself in internal movement and subjected to all the reactions of external movement, perceives this movement doubtless as an unique impression only. But when several cells come together and live in permanent contact, the impressions of external movement begin to be perceived as differentiated. Is this then necessary? Do there then already exist luminous vibrations, different from sonorous vibrations? Assuredly, since otherwise the sensorial differentiation would be inexplicable, being useless. The union of several cells permits the animal to divide its work of perception, and to present to each perceptible manifestation an organ or, at least, the sketch of an organ, appropriate to receive it. It would be possible, it is true, from the idealistic point of view, to suppose that the senses are a creation of the individual, an enhancement of his own life, and that he differentiates, on his own initiative, his cinematic impression. This would be a phenomenon of spontaneous analytic creation, the analyzing instrument existing prior to the matter analyzed, or even, for exasperated idealists, creating this matter according to determinate needs, once and for all, by its own physiology. It would then be a property of organized living matter to fabricate senses for itself, and to diversify, by this means, its own life. This point of view is not easy to admit for several reasons, purely physical. First, if this sensorial differentiation were a faculty of living matter, it would not be observed to be limited in its powers, as it is. Even admitting certain senses unknown to man, such as the chemical sense, the electric sense, the sense of orientation (extremely doubtful), it is still seen that the number of senses is very limited. But, far more important, the fundamental senses are found to be identical among the majority of the higher species, vertebrates and insects, with very few exceptions. The moment the animal arrives at sensorial differentiation, this differentiation occurs in response to the manifestations of matter.