But these philosophers took a further view of the question which especially applied to moral evil. They considered that nothing could be more groundless than to suppose that if there were no evil there could be any good in the world; and they illustrated this position by asking how we could know anything of temperance, fortitude or justice, unless there were such things as excess, cowardice and injustice.
These were the doctrines of the Stoics, from whose sublime and impracticable philosophy they seemed naturally enough to flow. Aulus Gellius relates that the last-mentioned argument was expounded by Chrysippus, in his work upon providence. The answer given by Plutarch seems quite sufficient: “As well might you say that Achilles could not have a fine head of hair unless Thersites had been bald; or that one man’s limbs could not be all sound if another had not the gout.”
In truth, the Stoical doctrine proceeds upon the assumption that all virtue is only the negative of vice; and is as absurd, if indeed it be not the very same absurdity, as the doctrine which should deny the existence of affirmative or positive truths, resolving them all into the opposite of negative propositions. Indeed, if we even were to admit this as an abstract position, the actual existence of evil would still be unnecessary to the idea, and still more to the existence, of good. For the conception of evil, the bare idea of its possibility, would be quite sufficient, and there would be no occasion for a single example of it.
The other doctrine, that of two opposite principles, was embraced by most of the other sects, as it should seem, at some period or other of their inquiries. Plato himself, in his later works, was clearly a supporter of the system; for he held that there were at least two principles, a good and an evil; to which he added a third, the moderator or mediator between them.
Whether this doctrine was, like many others, imported into Greece from the East, or was the natural growth of the schools, we cannot ascertain. Certain it is that the Greeks themselves believed it to have been taught by Zoroaster in Asia, at least five centuries before the Trojan war; so that it had an existence there long before the name of philosophy was known in the western world.