Ancient Chinese military strategist Sun Tzu dealt with the topic of spies extensively in his book The Art of War. He believed that information and intelligence determined the flow of war and spies were a vital element. Sun Tzu talked about five types of spies.
- Local spies (鄕間): Use the enemy’s people
- Internal spies (內間): Use the enemy’s officials (like a resident spy)
- Double spies (反間): Use the enemy’s spies to feed the wrong information
- Dead spies (死間): Has a possibility of betraying, so use them to spread misinformation, leading the enemy to persecute them
- Living spies (生間): Use agents that can gather intelligence and return safely back to report their findings, the most useful type of spies