To illustrate these observations let us briefly analyze the physical principles of our actions. When an object strikes any of our senses, and the sensation it produces is agreeable, it creates a desire, which desire must have a relation to some of our qualities or modes of enjoyment. The object we cannot desire but either to see, taste, hear, smell, or to touch. We desire it merely that we may render the first sensation still more agreeable, or to excite another which is a new manner of enjoying the object; for if in the moment that we perceive an object we could enjoy it fully, through all the senses at once, we should have nothing to desire. The source of desire, then, is our being badly situated with respect to the object perceived, our being either too far from, or too near to it. This being the case we naturally change our situation, because at the same time that we perceive the object, we likewise perceive the cause which prevents our obtaining a full enjoyment of it. From the impression which the object produces upon our senses, then, the motion we make in consequence of that desire, and the desire itself, solely proceeds.
An object we perceive by the eye, and which we desire to touch, if within our reach, we stretch forth our hands, and if at a distance we put ourselves in motion to approach it. A man deeply immersed in thought, if he is hungry, and there is a piece of bread before him, he will seize it, and even carry it to his mouth and eat it, without being conscious that he has done so. These movements are a necessary consequence of the first impressions of objects, and would never fail to succeed this impression if other intervening impressions did not often oppose this natural effect, either by weakening or by destroying the action of the first.
An organized being void of sensation, as an oyster, whose sense of feeling is probably very imperfect, is deprived not only of progressive motion, but even of sentiment and intelligence, as either of these would produce desire, which would manifest itself by exterior movement. That such beings are divested of a sense of their own existence I will not assert, but at least that sense must be very imperfect, since they have no perception of the existence of others.
It is the action of objects upon the senses which creates desire, and desire progressive motion. In order to render this truth still more sensible, let us suppose a man, at the instant his will incites him to approach an object, suddenly deprived of all his members, his body reduced to a physical point, to a globular atom, and, provided the desire still subsists, he will exert his whole strength in order to change his situation. The exterior and progressive movement depends not, then, upon the organization and figure of the body and members, since whatever be the conformation any of being it will not fail to move, provided it has senses, and a desire to gratify them.
On this exterior organization, indeed, depends the facility, quickness, direction, and continuity of motion, but the cause, principle, action, and determination, originate solely from desire occasioned by the impression of objects upon the senses; and if a man was deprived of them he would no longer have desire, and consequently remain constantly at rest, notwithstanding he might possess the faculties for motion.
The natural wants, as that of taking nourishment, are interior movements, which necessarily create desire or appetite. By these movements exterior motions may be produced in animals, and, provided they are not deprived of exterior senses relative to these wants, they will act to satisfy them. Want is not desire; it differs from it as the cause differs from the effect. Every time the animal perceives an object, relative to its wants, desire begins, and action follows.
The action of external objects must produce some effect; and this effect we readily conceive to be animal motion, as every time its senses are struck in the same manner, the same movements always follow. But how shall we comprehend the action of objects creating desire or aversion? How shall we obtain knowledge of that which operates beyond the senses, those being the intermediate between the action of objects, and the action of the animal; a power in which consists the principle of the determination of motion, since it modifies the action of the animal, and renders it sometimes null, notwithstanding the impression of objects?
This question, as it relates to man, is difficult to be resolved, being by nature so different from other animals. The soul has a share in all our movements, and to distinguish the effects of this spiritual substance, from those produced by the powers of our material being alone, is an object of very great difficulty, and of which we can form no judgment but by analogy, and by comparing our actions with the natural operations of other animals. But as man alone is possessed of this spiritual substance, which enables him to think and reflect, and as the brute is a being altogether material, which neither thinks nor reflects, nevertheless acts, and seems to determine, we cannot doubt but that the principle of the determination of motion is in the animals an effect altogether mechanical, and absolutely dependant upon its organization.
I conceive, therefore, that in the animal the action on objects on the senses produces another on the brain, which I consider as an interior and a general sense, which receives every impression that the exterior senses transmit to it. This internal sense is not only capable of being agitated by the action of the senses, but also of retaining for a length of time the agitations thus produced; and in the continuity of the agitation consists the impression, which is more or less deep in proportion as the agitation is more or less durable.