Albert Barnes on James 3:12

Can the fig-tree, my brethren, bear olive berries? Such a thing is impossible in nature, and equally absurd in morals. A fig-tree bears only figs; and so the tongue ought to give utterance only to one class of sentiments and emotions. These illustrations are very striking, and show the absurdity of that which the apostle reproves. At the same time, they accomplish the main purpose which he had in view, to repress the desire of becoming public teachers without suitable qualifications. They show the power of the tongue; they show what a dangerous power it is for a man to wield who has not the proper qualifications; they show that no one should put himself in the position where he may wield this power without such a degree of tried prudence, wisdom, discretion, and piety, that there shall be a moral certainty that he will use it aright.

(a) "Can the fig-tree...bear olive berries" Mt 7:16