The first division of the animal economy appears general and well founded. An animal when asleep is more easy to be examined than when awake and in motion. This difference is essential, and not a simple change of situation as in an inanimate body, which may be equally and indifferently at rest or in motion; for in either of these states it would perpetually remain, unless constrained to quit it by some external power or resistance. By its own powers the animal changes its condition; and naturally, and without constraint, it passes from repose to action, and from action to repose. The period for awaking returns as necessarily as that for sleep, and both arrive independent of any foreign cause; since in either state the animal cannot exist but for a certain time, and an uninterrupted continuity of either would be equally fatal, to life. In the animal economy, therefore, we may distinguish two parts; the one acts perpetually without interruption, and the other acts only by intervals. The action of the heart and lungs in animals that breathe, and of the heart in the foetus, seem to constitute the former as does the action of the senses, and the movements of the members of the latter. If we imagine beings endowed by nature with only the first part of this animal economy, though deprived of sense and progressive motion, would yet be animated, and differ in nothing from animals asleep. An oyster which appears to have no external sense or progressive motion, is a being formed to sleep for ever. In this sense a vegetable is merely a sleeping animal, and in general every organized being destitute of sense and motion may be compared to an animal doomed by Nature to a perpetual sleep. In animals, then, sleep is not an accidental state, occasioned by the exertions of their functions while awake. It is, on the contrary, an essential mode of existence, which serves as a basis to an animal economy. By sleep our existence begins; the foetus sleeps continually, and the infant is more often asleep than awake. Sleep, therefore, which seems to be a state purely passive, resembling that of death, is, on the contrary, that which a living animal first experiences, and is the very foundation of life.