Sun Tzǔ said: Raising a host of a hundred thousand men and marching them great distances entails heavy loss on the people and a drain on the resources of the State. The daily expenditure will amount to a thousand ounces of silver.724 There will be commotion at home and abroad, and men will drop down exhausted on the highways.725 As many as seven hundred thousand families will be impeded in their labour.726

Hostile armies may face each other for years, striving for the victory which is 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 in honors and emoluments,727 is the height of inhumanity.728

One who acts thus is no leader of men, no present help to his sovereign,729 no master of victory.730

Thus, what enables the wise sovereign and the good general to strike and conquer, and achieve things beyond the reach of ordinary men, is foreknowledge.731

Now this foreknowledge cannot be elicited from spirits;732 it cannot be obtained inductively from experience,733 nor by any deductive calculation.734

Knowledge of the enemy’s dispositions can only be obtained from other men.735

Hence the use of spies, of whom there are five classes: (1) Local spies; (2) inward spies; (3) converted spies; (4) doomed spies; (5) surviving spies.

When these five kinds of spy are all at work, none can discover the secret system.736 This is called737 “divine manipulation of the threads.”738 It is the sovereign’s most precious faculty.739

Having local spies740 means employing the services of the inhabitants of a district.741

Having inward spies, making use of officials of the enemy.742

Having converted spies, getting hold of the enemy’s spies and using them for our own purposes.743

Having doomed spies, doing certain things openly for purposes of deception, and allowing our spies to know of them and report them to the enemy.744

Surviving spies, finally, are those who bring back news from the enemy’s camp.745

Hence it is that which none in the whole army are more intimate relations to be maintained than with spies.746 None should be more liberally rewarded.747 In no other business should greater secrecy be preserved.748

Spies cannot be usefully employed749 without a certain intuitive sagacity.750

They cannot be properly managed without benevolence and straightforwardness.751

Without subtle ingenuity of mind, one cannot make certain of the truth of their reports.752

Be subtle! be subtle!753 and use your spies for every kind of business.

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.754

Whether the object be to crush an army, to storm a city, or to assassinate an individual, it is always necessary to begin by finding out the names of the attendants,755 the aides-de-camp,756 and doorkeepers and sentries757 of the general in command.758 Our spies must be commissioned to ascertain these.759

The enemy’s spies who have come to spy on us must be sought out,760 tempted with bribes, led away and comfortably housed.761 Thus they will become converted spies and available for our service.

It is through the information brought by the converted spy that we are able to acquire and employ local and inward spies.762

It is owing to his information, again, that we can cause the doomed spy to carry false tidings to the enemy.763

Lastly, it is by his information that the surviving spy can be used on appointed occasions.764

The end and aim of spying in all its five varieties is knowledge of the enemy;765 and this knowledge can only be derived, in the first instance, from the converted spy.766 Hence it is essential that the converted spy be treated with the utmost liberality.

Of old, the rise of the Yin dynasty767 was due to I Chih768 who had served under the Hsia. Likewise, the rise of the Chou dynasty was due to Lu Ya769 who had served under the Yin.770

Hence it is only the enlightened ruler and the wise general who will use the highest intelligence of the army for purposes of spying771 and thereby they achieve great results.772 Spies are a most important element in war, because on them depends an army’s ability to move.773