“Generally speaking, you’ll find no faculty that has the ability to examine itself, and therefore none that has the ability to assess itself and see whether or not it’s acceptable. …
If you’re writing to a friend, grammar will tell you how best to express yourself, but it won’t tell you whether or not you should be writing to your friend. …
Which faculty, then, will give you this information? The one that examines both itself and everything else. And which is that? The faculty of reason. …
Is there anything else that decides about musicology, grammar, and all the other faculties, that assesses their uses and shows when it’s the right time for them? No, there’s not.” (Epictetus, Discourses, I.1.1 to I.1.6)
Epictetus on the faculty of reason