However this may be, the sensibility of the brain, and all its parts, is a fact: if it be asked, whence comes this property? We shall reply, it is the result of an arrangement, of a combination, peculiar to the animal: it is thus that milk, bread, wine, change themselves in the substance of man, who is a sensible being: this insensible matter becomes sensible, in combining itself with a sensible whole. Some philosophers think that sensibility is a universal quality of matter: in this case, it would be useless to seek from whence this property is derived, as we know it by its effects. If this hypothesis be admitted, in like manner as two kinds of motion are distinguished in Nature, the one called _live_ force, the other _dead_, or _inert_ force, two sorts of sensibility will be distinguished, the one active or alive, the other inert or dead. Then to animalize a substance, is only to destroy the obstacles that prevent its being active or sensible. In fact, sensibility is either a quality which communicates itself like motion, and which is acquired by combination; or this sensibility is a property inherent in all matter: in both, or either case, an unextended being, without parts, such as the human soul is said to be, can neither be the cause of it nor submitted to its operation; but we may fairly conclude, that all the parts of Nature enjoy the capability to arrive at animation; the obstacle is only in the state, not in the quality. Life is the perfection of Nature: she has no parts which do not tend to it--which do not attain it by the same means. Life in an insect, a dog, a man, has no other difference, than that this act is more perfect, relatively to ourselves in proportion to the structure of the organs: if, therefore, it be asked, what is requisite to animate a body? we reply, it needs no foreign aid; it is sufficient that the power of Nature be joined to its organization.
The conformation, the arrangement, the texture, the delicacy of the organs, as well exterior as interior, which compose men and animals, render their parts extremely mobile, or make their machine susceptible of being moved with great facility. In a body, which is only a heap of fibres, a mass of nerves, contiguous one to the other, united in a common center, always ready to act; in a whole, composed of fluids and solids, of which the parts are in equilibrium, the smallest touching each other, are active in their motion, communicating reciprocally, alternately and in succession, the impression, oscillations, and shocks they receive; in such a composition, it is not surprising that the slightest impulse propagates itself with celerity; that the shocks excited in its remotest parts, make themselves quickly felt in the brain, whose delicate texture renders it susceptible of being itself very easily modified. Air, fire, water, agents the most inconstant, possessing the most rapid motion, circulate continually in the fibres, incessantly penetrate the nerves: without doubt these contribute to that incredible celerity with which the brain is acquainted with what passes at the extremities of the body.