Whilst all intelligence work is, by its very nature, somewhat uncertain, the world of counter-intelligence is even more fraught with complications. In the words of one US journalist, it is the ‘wilderness of mirrors’, where facts can be interpreted in various, often contrasting ways, depending on the starting assumption. Frequently conflated with counter-espionage in the academic literature, the art of counter-intelligence is to protect the integrity of your intelligence from counter-measures by the adversary: domestically, therefore, it focuses on those inside your own service who are surreptitiously working for the other side. Sometimes this can refer to your own nationals; sometimes it refers to foreigners who offer their services but whose loyalty might be questionable. The 1947 National Security Act provided a starting point in the US for such endeavours:

Counter-espionage has been described by one former CIA officer in a practical way: ‘Successful counterespionage brings with it new or enhanced knowledge of the adversary. When a spy is found, a service may observe his activities and learn how the other side runs him, or may double him and begin gathering information that way. When a spy is arrested and confesses (as most do), his interrogations will yield a wealth of information about the other side, as well as lessons for his own.’ Verifying authenticity is often based on a very simple principle: what is already known about the subject and can therefore be checked. Following this, a variety of other factors come into play, often based on psychology. The CIA has historically also relied upon the polygraph test as a means of assessing the veracity of subjects, though it is not foolproof. Counter-intelligence and counter-espionage work, history shows, can breed paranoia. Within the annals of counter-intelligence there is no figure more notorious than the long-serving head of CIA's counter-intelligence staff, James Jesus Angleton.