Why not? You saw Macy and Gimbel shaking hands. That wasn't possible either, but it happened.

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

You can hear this line at 01:10:55 in the Blu-ray version of the movie.

Quote context

[...]

- Read all about it! Gailey throws bombshell in New York Supreme Court.

- Yes, I read that.

- Oh, I didn't see this... Front page. Good, good.

- You're not serious about this.

- Of course I am.

- But you can't possibly prove that he's Santa Claus.

- Why not? You saw Macy and Gimbel shaking hands. That wasn't possible either, but it happened.

- Honestly, Fred...

- It's the best defense I can use - completely logical and completely unexpected.

- And completely idiotic.

- What about your bosses, Haislip and Mackenzie and the rest of them? What do they say?

- Hmm. That I am jeopardizing the prestige and dignity of an old, established law firm, and either I drop this impossible case immediately or they will drop me.

[...]