Now, since there are several types of cause (for we say that that for the sake of which, and that whence the origin of motion comes, and the matter, and the account, are all causes), in the first place, then, as we assert that nature operates for the sake of an end, and that this end is a good; and that to every creature which is endowed by nature with the power to move, but cannot with pleasure to itself move always and continuously, rest is necessary and beneficial; and since, taught by truth itself, men apply to sleep this metaphorical term, calling it a rest