Adam Clarke on Proverbs 14:10

The heart knoweth his own bitterness - מרת נפשו morrath naphsho, "The bitterness of its soul." Under spiritual sorrow, the heart feels, the soul feels; all the animal nature feels and suffers. But when the peace of God is spoken to the troubled soul, the joy is indescribable; the whole man partakes of it. And a stranger to these religious feelings, to the travail of the soul, and to the witness of the Spirit, does not intermeddle with them; he does not understand them: indeed they may be even foolishness to him, because they are spiritually discerned.