Metaphysics

Metaphysics was a branch of philosophy which included, among other things, causality and the question of the improbability of one's own existence. (PROSE: Separation Day) Miss Quill thought that metaphysics were not real; only thoughts. (TV: The Metaphysical Engine, or What Quill Did) Emily Rutherford thought it was just an idea, which didn't correspond to anything real. The Third Doctor identified this as a natural mistake, "but it's a serious one".

The metaphysical engine allowed for travel to locations based on thought alone, rather than actual physical space. (TV: The Metaphysical Engine, or What Quill Did) The TARDIS was also a metaphysical engine, capable of travelling into the dimension of a person's mind, their imagination. (PROSE: Every Day)