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)
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)
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)
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)