That which was at the origin of mankind is repeated at the birth of every human being; he comes into the world in a lethargic condition, but endowed with latent instincts which we name in one word, sense; common to man and to animals, it places them in relationship with the things exterior to themselves; this sense, or capability of sensation, is merely the general faculty of feeling. No newly-born child would emerge from its torpor if it were not surrounded by material objects which affirm their presence by reacting on him; his first act, at the moment when he perceives his surroundings is the transference of his own mind, until now isolated in itself alone, towards the objects which solicit his attention. The sense which operates in each child is inward, we name it briefly--sensation--to distinguish it from the five external senses, which are more familiar to us, since even at school their functions and modes of action have been explained to us. For instance, we know that it is only necessary to touch the strings of an instrument to cause them to vibrate, the vibrations are communicated to the air, and are then called waves of sound; they diffuse themselves with an incredible swiftness in space, advancing and retreating in the manner of the waves of the sea, they reach our ears, touch the auditory nerve, cause the tympanum to vibrate, penetrate to the brain, and give us instantaneously the sensation of sound. And it is to the waves of light passing through the ether, and communicating with the optic nerve of the organ of sight, that we owe the sensation of sight of the objects before us. The vacant look of a newly-born infant, implies that it has undergone an experience, it has felt something of the nature of a shock; a shock always implies resistance and yielding. In the child it is the human eye becoming conscious of itself amidst the impressions produced on it by the confused sight of external objects, and hearing the noises which occur around him. This instance is analogous to the vibratory movement of the waves described and even drawn in all manuals on physics. It is strange that a natural phenomenon which learned men have taken some trouble to analyse, should find expression in the following commonplace phrase. “From the clash of opinions light is generated.” If this phrase were not only on our lips, but also implanted in our mind, we should more readily have grasped the physiological fact of sensation. Sensation plays such an important part in the world of humanity, that all the sciences, both physical and moral, deal with it; but we, who grumble so readily and continuously at feeling either too hot or too cold, probably never enquire what philosophy has to do with purely bodily impressions.