The heaven for height, and the earth for depth, and the heart of kings is unsearchable.
– Proverbs 25:3
This verse uses a triplet that likens three things as being unsearchable. The point of the comparison is to focus on the heart of kings. The word for unsearchable is related to the word for search out in the previous verse, which ties the verses together thematically. The heaven for height and the earth for depth are essentially unfathomable and are put for something not fully knowable. The wisdom here is teaching servants and courtiers to be cautious in presuming to know the full mind of a king, or in being overconfident of his favor (Proverbs 16:14; 20:2; 24:21-22).