“We can’t move. We can’t investigate because all the instruments of enquiry are in the Circus’s hands, perhaps in the mole Gerald’s. We can’t watch, or listen, or open mail. … We can’t interrogate; we can’t take steps to limit a particular person’s access to delicate secrets. To do any of these things would be to run the risk of alarming the mole. It’s the oldest question of all, George. Who can spy on the spies? Who can smell out the fox without running with him?” — John Le Carre, Tinker, Tailor, Soldier, Spy
