13. The Use of Spies
Hence it is only the enlightened ruler and the wise general who will use the highest intelligence of the army for purposes of spying and thereby they achieve great results.
Tu Mu closes with a note of warning: "Just as water, which carries a boat from bank to bank, may also be the means of sinking it, so reliance on spies, while production of great results, is oft-times the cause of utter destruction."
Spies are a most important element in water, because on them depends an army's ability to move.
Chia Lin says that an army without spies is like a man without ears or eyes.