The lips of the wise disperse knowledge: but the heart of the foolish doeth not so.
– Proverbs 15:7
[T]he contrast between lips and heart causes difficulty among commentators. The heart is put for the mind and lips obviously refer to speech. There is a similar contrast in Proverbs 10:20. The wise will spread knowledge with their lips because they have wisdom (Proverbs 14:18). The heart, or mind, of the foolish is not filled with wisdom, so they obviously cannot speak or spread wisdom (Proverbs 10:21). Fools pursue folly and take delight in it (Proverbs 15:21). He takes in folly like a panting dog licking up water and dirt (Proverbs 15:14). He in turn pours out folly because that is what he has (Proverbs 13:16; 15:2).