...as if he were his own enemy lying in ambush

Epictetus, Enchiridion, Chapter 48:

The mark and attitude of the ordinary man: never look for help or harm from yourself, only from outsiders. The mark and attitude of the philosopher: look for help and harm exclusively from yourself.

And the signs of a person making progress: he never criticizes, praises, blames or points the finger, or represents himself as knowing or amounting to anything. If he experiences frustration or disappointment, he points the finger at himself. If he is praised, he is more amused than elated. And if he is criticized, he won't bother to respond. He walks around as if he were an invalid, careful not to move a healing limb before it's at full strength. He has expunged all desire, and made the things that are contrary to nature and in his control the sole target of his aversion. Impulse he only uses with detachment. He does not care if he comes across as stupid or naive. In a word, he keeps an eye on himself as if he were his own enemy lying in ambush.