The Thirteenth Introduction to the Academy: The Thirteenth "Jianpian" discusses the importance of using spies to detect the enemy's situation in combat, and discusses the types of spies and the methods of using spies. Sun Wu attached great importance to the role of spies and believed that they were a key to victory in combat. The army relied on the intelligence provided by spies to take action. Sun Wu divided spies into five types: the cause, the inner, the counter, the dead, and the living. The so-called cause is to use ordinary people in the enemy's country as spies. The so-called internecine means using officials of the enemy country as spies. The so-called counter-intelligence is to use the enemy's spies to my advantage. The so-called dead time is to deliberately spread false information to the outside world, let our spies know, and then pass it on to the enemy. The so-called living beings are spies who are sent to the enemy to reconnoiter the enemy's situation and can come back to report in person. Of these five types of spies, the first three use enemy personnel, and the latter two are used by us to infiltrate into the enemy. When these five types of spies are used, the sources of intelligence will be very wide. When fighting a war, the enemy will be at a loss and not know how to deal with it. It is indeed unpredictable. The information obtained from countermeasures is the most important. Therefore, special attention should be paid to counter-individuals and their treatment should be particularly generous.