War is a risky undertaking that consumes significant resources and impacts many lives. In face of the risks involved, an army makes her moves based on perceived advantages (军争为利，军争为危), which are informed ultimately by intelligence gathered. An enlightened general thus invests resources to gather accurate intelligence in order to reduce unnecessary destruction, casualties and sufferings of her people.
Intelligence can be best obtained from spies.
- 乡 is translated as “country” or “rural” and 乡间 refers to local inhabitants turned spies, who could provide local based knowledge
- 内 is translated as “internal” and 内间 refers to enemies turned spies, who are still employed by the enemy and could provide intelligence from within enemy forces.
- 反 translates as “turn” or “switch” and 反间 refers to enemy spies who switched sides, and could provide useful information about the enemy or misinform the enemy about one’s situation
- 死 translates as “dead” and 死间 refers to one’s spies sent to gain the trust of, and misinform the enemy about one’s situation, at the risk of death if exposed.
- 生 translates as “live” and 生间 refers to spies who returned from within the enemy with intelligence gathered about her.
These identities are not mutually exclusive. The concurrent use of all five types of spies, some with multiple identities, make for a complex and unfathomable intelligence network.
孙子曰：
凡兴师十万，出征千里，百姓之费，公家之奉，日费千金；
内外骚动，怠于道路，不得操事者，七十万家。
Sun Tzu 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. There will be commotion at home and abroad, and men will drop down exhausted on the highways. As many as seven hundred thousand families will be impeded in their labor.相守数年，以争一日之胜，
而爱爵禄百金，不知敌之情者，
不仁之至也，
非人之将也，
非主之佐也，
非胜之主也。
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,
is the height of inhumanity.
One who acts thus is no leader of men,
no present help to his sovereign,
no master of victory.故明君贤将，所以动而胜人，成功出于众者，先知也。
先知者，
不可取于鬼神，
不可象于事，
不可验于度，
必取于人，知敌之情者也。
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.
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.故用间有五：
有乡间，
有内间，
有反间，
有死间，
有生间。
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.五间俱起，莫知其道，是谓神纪，人君之宝也。
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 sovereign’s most precious faculty.乡间者，因其乡人而用之。
内间者，因其官人而用之。
反间者，因其敌间而用之。
死间者，为诳事于外，令吾间知之，而传于敌间也。
生间者，反报也。
Having local spies means employing the services of the inhabitants of a district.
Having inward spies, making use of officials of the enemy.
Having converted spies, getting hold of the enemy’s spies and using them for our own purposes.
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.
Surviving spies, finally, are those who bring back news from the enemy’s camp.故三军之事，
莫亲于间，
赏莫厚于间，
事莫密于间。
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 business should greater secrecy be preserved.非圣智不能用间，
非仁义不能使间，
非微妙不能得间之实。
Spies cannot be usefully employed without a certain intuitive sagacity.
They cannot be properly managed without benevolence and straightforwardness.
Without subtle ingenuity of mind, one cannot make certain of the truth of their reports.微哉！微哉！无所不用间也。
Be subtle! be subtle! and use your spies for every kind of business.间事未发，而先闻者，间与所告者皆死。
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.凡军之所欲击，
城之所欲攻，
人之所欲杀，
必先知其守将，左右，谒者，门者，舍人之姓名，令吾间必索知之。
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, the aides-de-camp, and door-keepers and sentries of the general in command. Our spies must be commissioned to ascertain these.必索敌人之间来间我者，
因而利之，
导而舍之，
故反间可得而用也。
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.因是而知之，故乡间、内间可得而使也；
因是而知之，故死间为诳事，可使告敌。
因是而知之，故生间可使如期。
It is through the information brought by the converted spy 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.五间之事，主必知之，
知之必在于反间，
故反间不可不厚也。
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.
Hence it is essential that the converted spy be treated with the utmost liberality.昔殷之兴也，伊挚在夏；
周之兴也，吕牙在殷。
Of old, the rise of the Yin dynasty was due to I Chih who had served under the Hsia.
Likewise, the rise of the Chou dynasty was due to Lu Ya who had served under the Yin.故惟明君贤将，能以上智为间者，必成大功。
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.此兵之要，三军之所恃而动也。
Spies are a most important element in war, because on them depends an army’s ability to move.Translation by Lionel Giles
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