10. Let us then guard our hearts, let us guard our mouths. Both have been written about. In this place we are bidden to take heed to our mouth; in another place thou art told: “Keep thy heart with all diligence.” If David took heed, wilt thou not take heed? If Isaiah had unclean lips—who said: “Woe is me, for I am undone, for I am a man, and have unclean lips”—if a prophet of the Lord had unclean lips, how shall we have them clean?