There are no dangerous thoughts for the simple reason that thinking itself is such a dangerous enterprise.
Hannah Arendt, interview with Roger Errera (October 1973), broadcast on Un Certain Regard (6 July 1974), ORTF TV, France. Published as "The Last Interview" in the Hannah Arendt Newsletter #2 (December 1999) and Hannah Arendt: The Last Interview and Other Conversations (Brooklyn: Melvin House, 2013).
Widely paraphrased as "There are no dangerous thoughts; thinking itself is dangerous."