We have now stated the cause of sleeping, viz., that it consists in the recoil by the corporeal element, borne upwards by the connatural heat, in a mass upon the primary sense organ; we have also stated what sleep is, having shown that it is a seizure of the primary sense organ, rendering it unable to actualize its powers; arising of necessity (for it is impossible for an animal to exist if the conditions which render it an animal be not fulfilled),.e., for the sake of its conservation; since remission of movement tends to the conservation of animals.