As I made my way through Portal 2, I came across this amusing sign (click for full-size image):

I was especially excited by the last line, because it is a reference to Russell’s Paradox, discovered by the British mathematician and logician Bertrand Russell in 1901.

Think of it this way: Suppose someone gave you a catalogue about catalogues, called SuperMegaCatalogue, that contained every cataglogue that had ever been printed. Would SuperMegaCatalogue be featured in the pages of SuperMegaCatalogue?

As the Wikipedia page puts it:

Let R be the set of all sets that are not members of themselves. If R qualifies as a member of itself, it would contradict its own definition as a set containing all sets that are not members of themselves. On the other hand, if such a set is not a member of itself, it would qualify as a member of itself by the same definition.

Or, to use the terms of symbolic logic:

So now the question is: Who’s more nerdy? The Portal 2 team, for including Russell’s Paradox in the game — or me, for spotting it and pointing it out to everyone else?

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