Albert Barnes Commentary Proverbs 25:3

"As the heavens for height, and the earth for depth, So the heart of kings is unsearchable." — Proverbs 25:3 (ASV)

This is the other side of the thought in Proverbs 25:2. As the mind of God is to those who search for knowledge, so the heart of the true and wise king is to those who try to discern its counsels.