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.

So can no fountain both yield salt water and fresh - For the reading of the common text, which is οὑτως ουδεμια πηγη ἁλυκον και γλυκυ ποιησαι ὑδωρ, so no fountain can produce salt water and sweet, there are various other readings in the MSS. and versions. The word οὑτως, so, which makes this a continuation of the comparison in James 3:11, is wanting in ABC, one other, with the Armenian and ancient Syriac; the later Syriac has it in the margin with an asterisk. ABC, five others, with the Coptic, Vulgate, one copy of the Itala, and Cyril, have ουτε ἁλυκον γλυκυ ποιησαι ὑδωρ, neither can salt water produce sweet. In the Syriac and the Arabic of Erpen, it is, So, likewise, sweet water cannot become bitter; and bitter water cannot become sweet. The true reading appears to be, Neither can salt water produce sweet, or, Neither can the sea produce fresh water; and this is a new comparison, and not an inference from that in James 3:11. This reading Griesbach has admitted into the text; and of it Professor White, in his Crisews, says, Lectio indubie genuina, "a reading undoubtedly genuine." There are therefore, four distinct comparisons here:
1. A fountain cannot produce sweet water and bitter.
2. A fig tree cannot produce olive berries.
3. A vine cannot produce figs.
4. Salt water cannot be made sweet. That is, according to the ordinary operations of nature, these things are impossible. Chemical analysis is out of the question.

Can the fig tree, my brethren, bear olive berries?.... Every tree bears fruit, according to its kind; a fig tree produces figs, and an olive tree olive berries; a fig tree does not produce olive berries, or an olive tree figs; and neither of them both:
either a vine, figs? or fig trees, grapes; or either of them, figs and grapes:
so can no fountain both yield salt water and fresh. The Alexandrian copy reads, "neither can the salt water yield sweet water"; that is, the sea cannot yield sweet or fresh water: the Syriac version renders it, "neither can salt water be made sweet": but naturalists say, it may be made sweet, by being strained through sand: the design of these similes is to observe how absurd a thing it is that a man should both bless and curse with his tongue.

Transition from the mouth to the heart.
Can the fig tree, &c.--implying that it is an impossibility: as before in James 3:10 he had said it "ought not so to be." James does not, as Matthew (Matthew 7:16-17), make the question, "Do men gather figs of thistles?" His argument is, No tree "can" bring forth fruit inconsistent with its nature, as for example, the fig tree, olive berries: so if a man speaks bitterly, and afterwards speaks good words, the latter must be so only seemingly, and in hypocrisy, they cannot be real.
so can no fountain . . . salt . . . and fresh--The oldest authorities read, "Neither can a salt (water spring) yield fresh." So the mouth that emits cursing, cannot really emit also blessing.