# Proof by legerdemain

Peli Grietzer shared a blog post by David Auerbach on Twitter yesterday containing the following lovely quote about Smullyan and Carnap:

I particularly delighted in playing tricks on the philosopher Rudolf Carnap; he was the perfect audience! (Most scientists and mathematicians are; they are so honest themselves ‘that they have great difficulty in seeing through the deceptions of others.) After one particular trick, Carnap said, “Nohhhh! I didn’t think that could happen in any possible world, let alone this one!”

In item # 249 of my book of logic puzzles titled What Is the Name of This Book?, I describe an infallible method of proving anything whatsoever. Only a magician is capable of employing the method, however. I once used it on Rudolf Carnap to prove the existence of God.

“Here you see a red card,” I said to Professor Carnap as I removed a card from the deck. “I place it face down in your palm. Now, you know that a false proposition implies any proposition. Therefore, if this card were black, then God would exist. Do you agree?”

“Oh, certainly,” replied Carnap, “if the card were black, then God would exist.”

“Very good,” I said as I turned over the card. “As you see, the card is black. Therefore, God exists!”

“Ah, yes!” replied Carnap in a philosophical tone. “Proof by legerdemain! Same as the theologians use!”

Raymond Smullyan, 5000 BC and Other Philosophical Fantasies. New York: St. Martin’s Press, 1983, p. 24.

See Auerbach’s post for more Carnap and Smullyan anecdotes.

# Why φ?

The most common alternative in use at the time was the use of Fraktur letters, e.g., $$\mathfrak{A}$$ as a metavariable for formulas, and A as a formula variable; x as a bound variable and $$\mathfrak{x}$$ as a metavariable for bound variables. This was the convention in the Hilbert school, also followed by Carnap. Kleene later used script letters for metavariables and upright roman type for the corresponding symbols of the object language. But indicating the difference by different fonts is perhaps not ideal, and Fraktur may not have been the most appealing choice anyway, both because it was the 1940s and because the type was probably not available in American print shops.