Isomorphic controls

Isomorphic controls are a security setting used in devices to prevent anyone other than the predetermined owner from accessing them. One known device which has isomorphic controls was the Master's laser screwdriver. (DW: Last of the Time Lords)

The TARDIS' controls were said to be isomorphic: that is, only the Doctor can operate them. (DW: Pyramids of Mars) However, various companions have been able to operate the TARDIS and even fly it. (DW: Castrovalva, Four to Doomsday, The Visitation, The Five Doctors, The Parting of the Ways, The Sontaran Stratagem, Journey's End, The Time of Angels) Even known enemies of the Doctor have been able to hijack the TARDIS (DW: Utopia), and the Time Lords are also able to pilot the TARDIS by remote control, usually, as the Doctor once bitterly noted, so he may take care of "some dirty work they don't want to get their lily-white hands on." (DW: Colony in Space, The Brain of Morbius) Whether this is because the isomorphic controls are not stable due to the TARDIS' age and outdatedness, as has been suggested for the status of its temporal grace field, or because the Doctor has at some point deactivated it for some reason or it has become broken, is unknown.