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.