Symbolic logic

Symbolic Logic was the basis of the Cybermen's control systems present in the Telos Cyber-tombs in order to test and provide Humans that possessed great inelligence, 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 was unable to work the controls unaided by The Doctor. The controls themselves consisted of a series of buttons and levers which must be pushed and pulled in the correct sequence to operate the Cyber-Technology present within the Tombs. (DW: 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 for all their control systems.