From St. Ambrose (The Nicene and Post-Nicene Fathers: Series Two Vol. X; Eerdmans pg. 57):  

'I know,' St. Paul says, 'how to be abased' (Phil. 4:12).

An untaught humility has no claim to praise, but only that which possesses modesty and a knowledge of self. For there is a humility that rests on fear, one too, that rests on want of skill and ignorance. Therefore Scripture says: 'He will save the humble in spirit' (Ps. 34:18). Gloriously, therefore, does he say: 'I know how to be abased;' that is to say, where, in what moderation, to what end, in what duty, in which office. The Pharisee knew not how to be abased, therefore he was cast down. The publican knew, and therefore he was justified (Lk. 18:11).