Symbolic logic

Symbolic Logic was an advanced form of higher mathematics. (AUDIO: Phantoms of the Deep)

It was the basis of the Cybermen's control systems in the Telos Cyber-tombs. It was there to test humans for their intelligence. They would then to be subjected to Cyber-conversion and become the new race of Cybermen.

The controls must have required a genius to solve them as Eric Klieg, a member of the Brotherhood of Logicians, could not work the controls without the Second Doctor. The controls consisted of a series of buttons and levers which had to be pushed and pulled in the correct sequence to operate the Cyber-Technology in the tombs. (TV: The Tomb of the Cybermen)

It remains unclear whether the Cybermen used symbolic logic purely for testing the intelligence of their "candidates" or if they used it for all their control systems.