Not necessarily.

4 seconds sound clip from the Miracle on 34th Street (1947) movie soundboard.

You can hear this line at 01:09:50 in the Blu-ray version of the movie.

Quote context

[...]

- I do, Your Honor. I'm fully aware of my client's opinions. In fact, that's the entire case against him. All these complicated tests and reports boil down to this.

- Mr. Kringle is not sane because he believes himself to be Santa Claus.

- An entirely logical and reasonable assumption, I'm afraid.

- It would be if the clerk here or Mr. Mara or I believed that we were Santa Claus.

- Anyone who thinks he's Santa Claus is not sane.

- Not necessarily.

- You believe yourself to be Judge Harper, yet no one questions your sanity because you are Judge Harper.

- I know all about myself, young man.

- Mr. Kringle is the subject of this hearing.

- Yes, Your Honor.

[...]