History of the Peloponnesian War

For of the gods we hold as a matter of opinion, and of men we know as a certainty, that, in obedience to an irresistible instinct, they always maintain dominion, wherever they are the stronger. And we neither enacted this law, nor were the first to carry it out when enacted; but having received it when already in force, and being about to leave it after us to be in force for ever, we only avail ourselves of it; knowing that both you and others, if raised to the same power, would do the same.