" It is essential to seek out enemy agents who have come to conduct espionage against you and to bribe them to serve you. Give them instructions and care for them. Thus doubled agents are recruited and used. It is by means of the doubled agent that native and inside agents can be recruited and employed. This is because the doubled agent knows those of his own countrymen who are covetous as well as those officials who have been remiss in office. These we can tempt into our service.

And it is by this means that the expendable agent, armed with false information, can be sent to convey it to the enemy. It is because doubled agents know in what respects the enemy can be deceived that expendable agents may be sent to convey false information. It is by this means also that living agents can be used at appropriate times.

The sovereign must have full knowledge of the activities of the five sorts of agents. This knowledge must come from the doubled agents, and therefore it is mandatory that they be treated with the utmost liberality.

And therefore only the enlightened sovereign and the worthy general who are able to use the most intelligent people as agents are certain to achieve great things. "

― Sun Tzu , The Art of War