Book VIII · 40
If thou takest away thy opinion about that which appears to give thee pain, thou thyself standest in perfect security.—Who is this self?—The reason.—But I am not reason.—Be it so. Let then the reason itself not trouble itself. But if any other part of thee suffers, let it have its own opinion about itself.
Take away thy opinion as to any imagined pain, and thou thyself art set in surest safety. What is 'thyself '? Reason. But I am not reason. Be it so. At all events let the Reason not cause itself pain, but if any part in thee is amiss, let it form its own opinion about itself.
Marcus Aurelius, Meditations VIII.40. Translated by George Long (1862). The Stoic Library.