Proverbs 20:27 - Matthew Poole's English Annotations on the Holy Bible

The spirit, i.e. the reasonable soul. Is the candle; is a clear and glorious light set up in man for his information and direction. Of the Lord; so called, partly because it comes from God in a more immediate and peculiar manner than the body doth, Ecclesiastes 12:7; and partly because it is in God's stead to observe and judge all a man's actions. Searching all the inward parts of the belly; discerning not only his outward actions, which are visible to others, but his most inward and secret thoughts and affections, which no other man can see, 1 Corinthians 2:11. The belly is here put for the heart, as it is frequently. The soul can reflect upon and judge of its own dispositions and actions; which plainly showeth that the heart is not so deceitful, but that a man by diligent study of it, and the use of the means appointed by God, may arrive at a certain knowledge of its state and condition, in reference to God and to salvation.