The Nicomachean Ethics written by Aristotle is an example of what kind of source?

Aristotle: Nicomachean Ethics Summary and Analysis of Book One Section 1:Every human action aims at some good, and the good which is chosen for its own sake rather than as means to an end is the highest good. Ethics is a part of politics, which is the most authoritative and architectonic science. An inquiry into ethics should not be expected to have the same sort of precision as a mathematical inquiry, because the nature of the subject-matter is different. A proper student of ethics must already have substantial life experience and training in virtue; otherwise he will not profit from the subject because he is more inclined to listen to his passions than to reason.