Pleasures unaccompanied by pain, on the other hand—and these are those derived from things naturally and not accidentally pleasant—do not admit of excess. By things accidentally pleasant I mean things taken as restoratives; really their restorative effect is produced by the operation of that part of the system which has remained sound, and hence the remedy itself is thought to be pleasant. Those things on the contrary are naturally pleasant which stimulate the activity of a given nature. 
Nothing however can continue to give us pleasure always, because our nature is not simple, but contains a second element （which is what makes us perishable beings）, and consequently, whenever one of these two elements is active, its activity runs counter to the nature of the other, while when the two are balanced, their action feels neither painful nor pleasant. Since if any man had a simple nature, the same activity would afford him the greatest pleasure always. Hence God enjoys a single simple pleasure perpetually. For there is not only an activity of motion: but also an activity of immobility, and there is essentially a truer pleasure in rest than in motion. But change in all things is sweet, as the poet says, owing to some badness in us; since just as a changeable man is bad, so also is a nature that needs change; for it is not simple nor good. 
We have now discussed the nature of Self-restraint and Unrestraint, and of Pleasure and Pain, and have shown in either case in what sense one of the two is good and the other evil. It remains for us to speak of Friendship.