Jamieson, Fausset and Brown on James 3:12

12. Transition from the mouth to the heart.

Can the fig tree, &c.--implying that it is an impossibility: as before in Jas 3:10 he had said it "ought not so to be." James does not, as Matthew (Mt 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.