SECT. XXV. Of Sleep.

What is more noble than a machine which continually repairs and renews itself? The animal, stinted to his own strength, is soon tired and exhausted by labour; but the more he takes pains, the more he finds himself pressed to make himself amends for his labour, by more plentiful feeding. Aliments daily restore the strength he had lost. He puts into his body another substance that becomes his own, by a kind of metamorphosis. At first it is pounded, and being changed into a liquor, it purifies, as if it were strained through a sieve, in order to separate anything that is gross from it; afterwards it arrives at the centre, or focus of the spirits, where it is subtilised, and becomes blood. And running at last, and penetrating through numberless vessels to moisten all the members, it filtrates in the flesh, and becomes itself flesh. So many aliments, and liquors of various colours, are then no more than one and the same flesh; and food which was but an inanimate body preserves the life of the animal, and becomes part of the animal himself; the other parts of which he was composed being exhaled by an insensible and continual transpiration. The matter which, for instance, was four years ago such a horse, is now but air, or dung. What was then either hay, or oats, is become that same horse, so fiery and vigorous--at least, he is accounted the same horse, notwithstanding this insensible change of his substance.

The natural attendant of food is sleep; in which the animal forbears not only all his outward motions, but also all the principal inward operations which might too much stir and dissipate the spirits. He only retains respiration, and digestion; so that all motions that might wear out his strength are suspended, and all such as are proper to recruit and renew it go on freely of themselves. This repose, which is a kind of enchantment, returns every night, while darkness interrupts and hinders labour. Now, who is it that contrived such a suspension? Who is it that so well chose the operations that ought to continue; and, with so just discernment, excluded all such as ought to be interrupted? The next day all past fatigue is gone and vanished. The animal works on, as if he had never worked before; and this reviving gives him a vivacity and vigour that invites him to new labour. Thus the nerves are still full of spirits, the flesh smooth, the skin whole, though one would think it should waste and tear; the living body of the animal soon wears out inanimate bodies, even the most solid that are about it; and yet does not wear out itself. The skin of a horse, for instance, wears out several saddles; and the flesh of a child, though very delicate and tender, wears out many clothes, whilst it daily grows stronger. If this renewing of spirits were perfect, it would be real immortality, and the gift of eternal youth. But the same being imperfect, the animal insensibly loses his strength, decays and grows old, because everything that is created ought to bear a mark of nothingness from which it was drawn; and have an end.