English translation
Sun Tzu said: In general, when mobilizing an army of a hundred thousand and campaigning a thousand li, the people's expenses and the state's provisions amount to a thousand pieces of gold a day; internal and external unrest follow, the people are exhausted along the roads, and seven hundred thousand households are unable to attend to their affairs. To wage war for years in pursuit of a single day's victory, yet begrudge a hundred pieces of gold in titles and rewards and so remain ignorant of the enemy's condition, is the height of inhumanity; such a person is not a commander of men, not a helper to his sovereign, and not a master of victory. Therefore an enlightened ruler and a worthy general who can move and defeat others, with success arising from foreknowledge, are those who act on prior knowledge. Foreknowledge cannot be obtained from ghosts or spirits, inferred from analogy, or verified by calculation; it must be obtained from people who know the enemy's situation.
Chinese source
孫子曰：凡興師十萬，出征千里，百姓之費，公家之奉，日費千金，內外騷動，怠于道路，不得操事者，七十萬家。相守數年，以爭一日之勝，而愛爵祿百金，不知敵之情者，不仁之至也，非人之將也，非主之佐也，非勝之主也。故明君賢將，所以動而勝人，成功出於眾者，先知也。先知者，不可取於鬼神，不可象於事，不可驗於度，必取於人，知敵之情者也。
Hence there are five kinds of spies: local spies, inside spies, turned spies, expendable (dead) spies, and returning (living) spies. When all five kinds of spies are active and no one knows their methods, this is called 'divine order' and is the sovereign's treasure. Local spies are used because they are countrymen; inside spies are used because they are officials; turned spies are used because they are enemy agents; expendable spies are used to spread false stories outside so that our spies learn them and then pass them to the enemy; returning spies are those who come back with reports.
Chinese source
故用間有五：有鄉間，有內間，有反間，有死間，有生間。五間俱起，莫知其道，是謂「神紀」，人君之寶也。鄉間者，因其鄉人而用之；內間者，因其官人而用之；反間者，因其敵間而用之；死間者，為誑事於外，令吾間知之，而傳於敵間也；生間者，反報也。
Therefore, of the affairs of the three armies nothing is more intimate than spies, nothing is awarded more richly than spies, and nothing must be kept more secret than spies. Without sage wisdom one cannot employ spies; without benevolence and righteousness one cannot make use of spies; without subtlety one cannot obtain true reports from spies. How subtle! How subtle! There is nothing in which spies are not employed. If an espionage matter has not yet begun but is heard of in advance, both the spy and those to whom he reported will die.
Chinese source
故三軍之事，莫親於間，賞莫厚於間，事莫密於間，非聖智不能用間，非仁義不能使間，非微妙不能得間之實。微哉！微哉！無所不用間也。間事未發而先聞者，間與所告者皆死。
Whenever the army intends to strike, a city to be taken, or men to be killed, one must first learn the names of the defending general, his attendants, ushers, gatekeepers, and household servants; our spies must be ordered to obtain these names. One must also seek out enemy agents who come to spy on us, reward them, guide them, and then release them; thus a turned spy can be obtained and put to use. Through this, local and inside spies can be identified and employed; through this, expendable spies can be made to convey falsehoods to the enemy; through this, returning spies can be made to report on schedule. The ruler must know the affairs of all five kinds of spies; such knowledge depends on turned spies, and therefore turned spies must be rewarded generously.
Chinese source
凡軍之所欲擊，城之所欲攻，人之所欲殺，必先知其守將、左右、謁者、門者、舍人之姓名，令吾間必索知之。必索敵人之間來間我者，因而利之，導而舍之，故反間可得而用也；因是而知之，故鄉間、內間可得而使也；因是而知之，故死間為誑事，可使告敵；因是而知之，故生間可使如期。五間之事，主必知之，知之必在於反間，故反間不可不厚也。
In former times, when the Shang arose, Yi Zhi served in Xia; when the Zhou rose, Lü Ya served in Shang. Therefore an enlightened ruler and a worthy general who can employ the wisest as spies will surely accomplish great deeds. This is the essence of warfare, the reliance of the three armies when they move.
Chinese source
昔殷之興也，伊摯在夏；周之興也，呂牙在殷。故明君賢將，能以上智為間者，必成大功。此兵之要，三軍之所恃而動也。