The Republic - pg. 176

"None at all," I said, "but this is the very charge I'm bringing; not one city today is in a condition worthy of the philosophic nature. And this is why it is twisted and changed; just as a foreign seed sown in alien ground s likely to be overcome and fade away into the native stock, so too this class does not at present maintain its own power but falls away into an alien disposition. But if it ever takes hold in the best regime, just as it is itself best, then it will make plain that it really is divine as we agreed it is and that the rest are human, both in terms of their nature and their practices. Of course, it's plain that next you'll ask what this regime is."