‘Can a fig tree, my brothers, yield olives, or a vine figs? Neither can salt water yield sweet.'
And he closes off the series with an illustration. Each thing in nature produces according to it nature. The fig tree produces figs, the olive tree olives. (Compare here Matthew 7:16). And salt water cannot produce sweet without treatment. So should the Christian from his mouth produce the good fruit of the Spirit (Galatians 5:22) and not the inedible and poisonous fruit of the flesh. The salt water probably has in mind the salt water springs around the Dead Sea which were unusable to man. Note that here the contrasts have ceased. The final word is intended to bring out the bitter saltiness of the tongue