P40: Things which are of assistance to the common Society of men, or which bring it about that men live harmoniously, are useful; those, on the other hand, are evil which bring discord to the State.

Dem.: For things which bring it about that men live harmoniously, at the same time bring it about that they live according to the guidance of reason (by P35). And so (by P26 and P27) they are good.

And on the other hand (by the same reasoning), those are evil which arouse discord, q.e.d.