Conceptual bomb

The conceptual bomb was a weapon developed by the Vess at the Factory. Planted inside an individual's brain, it made thought into reality and, subsequently, affected reality depending on the nature of the thought.

The Master planted a conceptual bomb in Bob Dovie's mind that would detonate upon him entering the Doctor's TARDIS on 23 November 1963 and believing the dimensional transcendence to be impossible. The bomb destroyed the TARDIS, removing it from existence so that the Doctor never chose that specific TARDIS and so never left Gallifrey. This timeline was eventually negated. (AUDIO: The Light at the End)