John Gill Commentary James 3:12

"Can a fig tree, my brethren, yield olives, or a vine figs? Neither [can] salt water yield sweet." — James 3:12 (ASV)

Can the fig tree, my brethren, bear olive berries ?
&c.] 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.