The parallelism of the two clauses is probably one of contrast. If so, the proverb is a comparison between all teaching from without and that of the light within. “The words of a man’s mouth” are dark as the “deep waters” of a pool, or tank (“deep waters” being associated in the Old Testament with the thought of darkness and mystery; compare Proverbs 20:5; Psalms 69:2; Ecclesiastes 7:24); but “the wellspring of wisdom is as a flowing brook,” bright and clear. The verse presents a contrast like that of Jeremiah 2:13.
Related Commentaries of Proverbs 18:4
Proverbs 18:4
4 The words of a man's mouth are as deep waters, and the wellspring of wisdom as a flowing brook.