What does Proverbs 4:23 mean?

To "keep" something, in this sense, means more than simply "to maintain ownership." It refers to maintenance, care, and support. Some translations use the word "guard," A wise person realizes that temptation is real (1 Corinthians 10:13) and takes appropriate precaution. This parallels statements made earlier, where Solomon advises his son to actively avoid evil, and those who practice it (Proverbs 4:14–15).

Scripture places a strong emphasis on the heart as the source of good or evil. Jeremiah 17:9 says, "the heart is deceitful above all things, and desperately sick." Jesus announced: "The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks" (Luke 6:45).

David, Solomon's father, certainly understood how influential the heart is in how we live out our morals and values. Out of the evil of his heart he stole Uriah's wife Bathsheba, committed adultery with her, and then arranged for Uriah's death when he could not hide his crime (2 Samuel 11:2–5, 14–15). However, when he was under heavy conviction about his evil deed, David prayed, "Create in me a clean heart, O God, and renew a right spirit within me" (Psalm 51:10). What is hidden in the heart is open to God's eyes. First Samuel 16:7 reminds us that "the LORD sees not as a man sees: man looks on the outward appearance, but the LORD looks on the heart."

It is absolutely essential, therefore, to guard the heart.