[Literally "visitors", is equivalent, as Tu Yu says, to "those whose duty it is to keep the general supplied with information," which naturally necessitates frequent interviews with him.]
the door-keepers and sentries of the general in command. Our spies must be commissioned to ascertain these.
[As the first step, no doubt towards finding out if any of these important functionaries can be won over by bribery.]
21. The enemy’s spies who have come to spy on us must be sought out, tempted with bribes, led away and comfortably housed. Thus they will become converted spies and available for our service.
22. It is through the information brought by the converted spy that we are able to acquire and employ local and inward spies.
[Tu Yu says: "through conversion of the enemy’s spies we learn the enemy’s condition." And Chang Yu says: "We must tempt the converted spy into our service, because it is he that knows which of the local inhabitants are greedy of gain, and which of the officials are open to corruption.">[
23. It is owing to his information, again, that we can cause the doomed spy to carry false tidings to the enemy.
[Chang Yu says, "because the converted spy knows how the enemy can best be deceived.">[
24. Lastly, it is by his information that the surviving spy can be used on appointed occasions.
25. The end and aim of spying in all its five varieties is knowledge of the enemy; and this knowledge can only be derived, in the first instance, from the converted spy.