He watches himself as if he were an enemy and lying in an ambush

From The Enchiridion by Marcus Aurelius

The condition and characteristic of an uninstructed person is this: he never expects from himself profit, or advantage, nor harm, but from externals.

The condition of a philosopher is this: he expects all advantage and all harm from himself. The signs of one who is making progress are these: he censures no man, he praises no man, he says nothing about himself as if he were somebody or knew something. When he is impeded or hindered, he blames himself. If a man praises him he ridicules the praiser to himself and if a man censures him, he makes no defence. He removes desires from himself, and transfers aversion to those things which are contrary to nature. He employs a moderate attitude towards everything; whether he is considered fooling or ignorant he cares not.

In a word, he watches himself as if he were an enemy and lying in an ambush.

Makes sense, since we are usually our own worst enemy.