17-18. The mind concerns itself with four things: these are good and evil, life and death. They all begin in the mind, but the tongue is their absolute ruler.
19. Someone may have the ability to teach, but still not be of much use to himself.
20. He may be clever with words, but end up starving because people hate him.
21. This may happen if he hasn't any sense, if the Lord has not given him tact.