Doth a fountain. He adduces these comparisons in order to shew that a cursing tongue is something monstrous, contrary to all nature, and subverts the order everywhere established by God. For God hath so arranged things which are contrary, that inanimate things ought to deter us from a chaotic mixture, sure as is found in a double tongue. [1]

Footnotes

1 - There is a different reading at the end of the James 3:12, adopted by Griesbach, though rejected by Mill and others: houtos oute haluchon gluchu poiosai hudor, "So neither can salt water produce sweet." This reading is favored by the Syr. and Vulg., though the words are somewhat different.

Doth a fountain send forth at the same place - Margin, "hole." The Greek word means "opening, fissure," such as there is in the earth, or in rocks from which a fountain gushes.
Sweet water and bitter - Fresh water and salt, James 3:12. Such things do not occur in the works of nature, and they should not be found in man.

Doth a fountain send forth - sweet water and bitter? - In many things nature is a sure guide to man; but no such inconsistency is found in the natural world as this blessing and cursing in man. No fountain, at the same opening, sends forth sweet water and bitter; no fig tree can bear olive berries; no vine can bear figs; nor can the sea produce salt water and fresh from the same place. These are all contradictions, and indeed impossibilities, in nature. And it is depraved man alone that can act the monstrous part already referred to.

Doth a fountain send forth at the same place,.... "Or hole"; for at divers places, and at different times, as Pliny (m) observes, it may send forth
sweet water and bitter: and it is reported (n), there is a lake with the Trogloditae, a people in Ethiopia, which becomes thrice a day bitter, and then as often sweet; but then it does not yield sweet water and bitter at the same time: this simile is used to show how unnatural it is that blessing and cursing should proceed out of the same mouth.
(m) Nat. Hist. l. 2. c. 103. (n) Isodor. Hispal. Originum, l. 13. c. 13. p. 115.

fountain--an image of the heart: as the aperture (so the Greek for "place" is literally) of the fountain is an image of man's mouth. The image here is appropriate to the scene of the Epistle, Palestine, wherein salt and bitter springs are found. Though "sweet" springs are sometimes found near, yet "sweet and bitter" (water) do not flow "at the same place" (aperture). Grace can make the same mouth that "sent forth the bitter" once, send forth the sweet for the time to come: as the wood (typical of Christ's cross) changed Marah's bitter water into sweet.