Book VII · 20
One thing only troubles me, lest I should do something which the constitution of man does not allow, or in the way which it does not allow, or what it does not allow now.
I am concerned about one thing only, that I of myself do not what man's constitution does not will, or wills not now, or in a way that it wills not.
Marcus Aurelius, Meditations VII.20. Translated by George Long (1862). The Stoic Library.