Theory

Whilst disguised in the skin suit of Lord Mayor of Cardiff Margaret Blaine, Blon Fel-Fotch Passameer-Day Slitheen dismissed Cathy Salt's suggestion that the Blaidd Drwg project was cursed and that the nuclear power plant could meltdown as "outlandish theories". (TV: Boom Town)

The Thirteenth Doctor believed the Solitract to be a theory. (TV: It Takes You Away)

In preparation for the role of Susan in Dr. Who and the Daleks, Roberta Tovey taught herself about the "Theory" behind TARDIS by reading the book Physics for the Inquiring Mind by Eric M. Rogers. (PROSE: Lady Penelope Investigates the stars of the Sensational new film Dr. Who and the Daleks!)