Raising a host of a hundred thousand men and engaging them in war entails heavy loss on the people and a drain on the resources. The daily expenditure will amount to a thousand ounces of silver. There will be commotion at home and abroad, and men will drop out exhausted.

Opposing forces may face each other for years, striving for the victory which may be decided in a single day. This being so, to remain in ignorance of the enemy's condition simply because one grudges the outlay of a hundred ounces of silver is the height of stupidity.

One who acts thus is no leader of men, no present help to his cause, no master of victory. Thus, what enables the wise commander to strike and conquer, and achieve things beyond the reach of ordinary men, is foreknowledge. Now this foreknowledge cannot be elicited from spirits; it cannot be obtained inductively from experience, nor by any deductive calculation. Knowledge of the enemy's dispositions can only be obtained from other men.

Hence the use of spies, of whom there are five classes:

(1) Local spies - Having local spies means employing the services of the inhabitants of an enemy territory;

(2) Moles - Having moles means making use of officials of the enemy;

(3) Double agents - Having double agents means getting hold of the enemy's spies and using them for our own purposes;

(4) Doomed spies - Having doomed spies means doing certain things openly for purposes of deception, and allowing our spies to know of them and report them to the enemy;

(5) Surviving spies - Surviving spies means are those who bring back news from the enemy's camp.

When these five kinds of spy are all at work, none can discover the secret system. This is called "divine manipulation of the threads." It is the commander's most precious faculty. Hence it is that which none in the whole army are more intimate relations to be maintained than with spies. None should be more liberally rewarded. In no other fields should greater secrecy be preserved.

(1) Spies cannot be usefully employed without a certain intuitive sagacity;

(2) They cannot be properly managed without benevolence and straight forwardness;

(3) Without subtle ingenuity of mind, one cannot make certain of the truth of their reports;

(4) Be subtle! be subtle! and use your spies for every kind of warfare;

(5) If a secret piece of news is divulged by a spy before the time is ripe, he must be put to death together with the man to whom the secret was told.

Whether the object be to crush an enemy, to storm a territory, or to kill an enemy general, it is always necessary to begin by finding out the names of the attendants, the aides-de-camp, and door-keepers and sentries of the general in command. Our spies must be commissioned to ascertain these.

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 double agents and available for our service. It is through the information brought by the double agent that we are able to acquire and employ local and inward spies. It is owing to his information, again, that we can cause the doomed spy to carry false tidings to the enemy.

Lastly, it is by his information that the surviving spy can be used on appointed occasions. 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 double agent. Hence it is essential that the double agent be treated with the utmost liberality.
Hence it is only the enlightened and wise general who will use the highest intelligence of the army for purposes of spying and thereby they achieve great results. Spies are the most important asset, because on them depends an army's ability to march.