The first and most important field of philosophy is....

“ The first and most important field of philosophy is the application of principles such as “Do not lie.” Next come the proofs, such as why we should not lie. The third field supports and articulates the proofs, by asking, for example, “How does this prove it? What exactly is a proof, what is logical inference, what is contradiction, what is truth, what is falsehood?” Thus, the third field is necessary because of the second, and the second because of the first. The most important, though, the one that should occupy most of our time, is the first. But we do just the opposite. We are preoccupied with the third field and give that all our attention, passing the first by altogether. The result is that we lie – but have no difficulty proving why we shouldn’t. ”

Epictetus

From : The Art of Living: The Classical Manual on Virtue