Reason

Reason was a logical method of decision making. As the Twelfth Doctor told Bill Potts to remember, "Passion fights, but reason wins." He suggested that diplomacy and tact were greater tools towards solving a problem than anger and outrage. (TV: Thin Ice ^[+]Loading...["Thin Ice (TV story)"])

Borusa wrote a paper on the topic of reason, which the Doctor had read. When Rodan told her that it suggested the Fourth Doctor was a traitor, Leela replied that "reason [was] a liar". (TV: The Invasion of Time ^[+]Loading...["The Invasion of Time (TV story)"])

The Twelfth Doctor suggested more than once that instead of steering the TARDIS, he had to reason with it, (TV: Smile) "unsuccessfully most of the time". (TV: Thin Ice ^[+]Loading...["Thin Ice (TV story)"])

In early 2006, the webmaster of Who is Doctor Who? was aware that there existed a reason for MP Joseph Green's recent weight gain. (PROSE: Rose sighting confirmed ^[+]Loading...["Rose sighting confirmed (short story)"])

Behind the scenes

In non-valid sources

The Green Arrow remarked to the Doctor that Jack Harkness looked familiar to him "for some reason". (GAME: LEGO Dimensions ^[+]Loading...["LEGO Dimensions (video game)"])