Proverbs 20:27 - Joseph Benson’s Commentary on the Old and New Testaments

The spirit of a man That is, the rational soul; is the candle, &c. Is a clear and glorious light, set up in man for his information and direction. It is said to be the candle of the Lord, because it comes from God in a more immediate manner than the body, Ecclesiastes 12:7; and because it is in God's stead, to observe and judge all our actions. Searching all the inward parts of the belly Discerning not only man's outward actions, which are visible to others, but his most inward thoughts and affections. 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; and by the use of the means which God hath appointed, especially the word of God, and prayer for supernatural light, may arrive at a certain knowledge of its state and condition, in reference to God and salvation.