What does Proverbs 20:5 mean?

Even in modern language, ideas which require clearer thought to understand are described as "deep." When something is sunken under water, it is harder to access, more difficult to see, and requires more effort to bring up. In this proverb, Solomon is using this imagery in one or more ways.

Human motives are not always clear, even to the person themselves. A wise person can draw them out of himself through thought and prayer. Or he can help another uncover them by counseling him. Even outside of counseling, motives, intentions, thoughts, and expectations can sometimes be discerned by a wise person. God has placed in the church gifted individuals who can assist others by leading them into a correct understanding of those inner aspects of motivation.

Everything that lies in a person's heart needs to conform to God's standards and plans. God does not leave us alone in this. In addition to wise counselors in the church, the Holy Spirit searches the heart and convicts believers of sin—whatever is contrary to God's standards and plans. He helps believers know the truth (John 14:26; 15:26; 16:13–15). Paul writes in 1 Corinthians 2:11–13: "For who knows a person's thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God. Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God. And we impart this in words not taught by human wisdom but taught by the Spirit, interpreting spiritual truths to those who are spiritual."