In truth, self knowledge seems the most important of all. For not only the eye, looking at outward things, fails to exercise its sight upon itself, but our understanding also, though very quick in apprehending the sin of another, is slow to perceive its own defects.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom. 9, in Hexameron.