The last chapter of a book about war ends with people rather than blades. Sun Tzu sorted the ways of getting information into five.
The five move in different places and by different means. And only when they are used together do they show their power.
用間有五:有因間,有內間,有反間,有死間,有生間
yong jian you wu, you yin jian, you nei jian, you fan jian, you si jian, you sheng jian
There are five kinds of spy. Local, inside, turned, doomed and living. (The Art of War, The Use of Spies, translated directly)