Theory

Staff Sergeant A. Frederick attempted to seed a false theory with the press to cover up Operation Mannequin, (PROSE: Operation Mannequin) the 2005 Auton invasion of London. (TV: Rose, et al.)

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!)

A group of historians who succeeded the Siege of Trenzalore studied Dalek history and developed several theories about unexplained aspects of the Daleks, such as the Eternal Dalek or the Pa Jass-Gutrik. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) The Dalek Combat Training Manual likewise had many Time Lord theories about Dalek aspects that they could not readily place along their foes' timeline along with many as to how the Time Lords might exploit Dalek vulnerabilities. (PROSE: Dalek Combat Training Manual)

A Time Lord author, in his textbook for the Time Lord Academy, offerred several theories about unexplained aspects of Gallifreyan history. (PROSE: A Brief History of Time Lords)

When the Twelfth Doctor and Ashildr met at the end of the universe, both offered their own theories about the Hybrid. The Doctor thought that the prophecy referred to the largely human and part-Mire Ashildr. For her part, she theorised that that the Hybrid was a metaphor that referred to union of the Doctor, a Time Lord, and Clara Oswald, a human. (TV: Hell Bent)