“People who are crazy rarely question whether they're crazy.”

Television, Inside the Actor's Studio (2006)

Nora Ephron, Heartburn (1983), as reported in What a piece of work is man!: Camp's unfamiliar quotations from 2000 B.C. to the present (1989), p. 320.

Said to Wolfgang Pauli after his presentation of Heisenberg's and Pauli's nonlinear field theory of elementary particles, at Columbia University (1958), as reported by F. J. Dyson in his paper “Innovation in Physics” (Scientific American, 199, No. 3, September 1958, pp. 74-82; reprinted in "JingShin Theoretical Physics Symposium in Honor of Professor Ta-You Wu," edited by Jong-Ping Hsu & Leonardo Hsu, Singapore; River Edge, NJ: World Scientific, 1998, pp. 73-90, here: p. 84).
Your theory is crazy, but it's not crazy enough to be true.
As quoted in First Philosophy: The Theory of Everything (2007) by Spencer Scoular, p. 89
There are many slight variants on this remark:
We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough.
We are all agreed that your theory is crazy. The question is whether it is crazy enough to be have a chance of being correct.
We in the back are convinced your theory is crazy. But what divides us is whether it is crazy enough.
Your theory is crazy, the question is whether it's crazy enough to be true.
Yes, I think that your theory is crazy. Sadly, it's not crazy enough to be believed.

Source: Veronika Decides to Die

"The Adopted Father", Isaac Asimov's Science Fiction Magazine (1980), Reprinted in Gene Wolfe, Gene Wolfe's Book of Days (1981), Reprinted in Gene Wolfe, Castle of Days (1992)
Fiction

Watching the inhabitants of the house trying to ward off the vampire
Dracula (1931)

Source: Veronika Decides to Die

Source: 2000s and posthumous publications, A Time Odyssey, Sunstorm (2005), Chapter 27, “The Tin Lid” (p. 207)

Source: Of Love and Other Demons