What does Psalm 139:23 mean?

Although David despised the wicked who spoke against God and took His name in vain, he realized he was not perfect. He was keenly aware that sin might be lurking in his heart and mind. Therefore, he asked the Lord to search his heart and know his thoughts. The Hebrew word translated "search" here is one often used to describe the way miners searched deeply for gold. This is the same term used at the beginning of this psalm (Psalm 139:1), as well as of Israelites scouring the land for territory to claim as their own (Judges 18:2).

David was a man after God's own heart (Acts 13:22), but he knew the heart is "deceitful above all things, and desperately sick" (Jeremiah 17:9). Only the omniscient God, whom he described earlier in the psalm, could search the heart (1 Samuel 16:7). David also asked the Lord to know his thoughts. Again, only the omniscient God could do this.

David wrote in Psalm 139:2 that God discerned his thoughts. Our thought lives matter. Thoughts form the seed bed of sin (James 1:14–15), therefore we ought to practice what Paul counseled in Philippians 4:8: "whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things."