wedrifid comments on Unsolved Problems in Philosophy Part 1: The Liar's Paradox - Less Wrong

4 Post author: Kevin 30 November 2010 08:56AM

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Comment author: cousin_it 30 November 2010 09:38:33AM *  16 points [-]

The formalist school of math philosophy thinks that meaningful questions have to be phrased in terms of finite computational processes. But if you try to write a program for determining the truth value of "this statement is false", you'll see it recurses and never terminates:

def f():
return (not f())

See also Kleene-Rosser paradox. This may or may not dissolve the original question for you, but it works for me.

There's more to be said about the paradox because it keeps turning up in many contexts. For example, see Terry Tao's posts about "no self-defeating object". Also note that if we replace "truth" with "provability", the liar's paradox turns into Godel's first incompleteness theorem, and Curry's paradox turns into Löb's theorem.

ETA: see also Abram Demski's explanation of Kripke's fixed point theory here on LW, if that's your cup of tea.

Comment author: wedrifid 30 November 2010 10:26:19AM *  3 points [-]

Also note that if we replace "truth" with "provability", the liar's paradox turns into Godel's first incompleteness theorem, and Curry's paradox turns into Löb's theorem.

The wikipedia link for Curry's paradox claims "It has also been called Löb's paradox after Martin Hugo Löb." Given that you require a word substitution I take it that wikipedia is oversimplyifying something? (Or perhaps overloading the Lob keyword at tad.)

Comment author: cousin_it 30 November 2010 10:37:24AM 4 points [-]

The two are related, so the overloading is probably not accidental. When I studied math we used to joke that every area of classical math has a Gauss theorem, and more often than not it's the most important theorem in the area.

Comment author: wedrifid 30 November 2010 10:41:46AM *  1 point [-]

The two are related, so the overloading is probably not accidental.

Not accidental and not surprising either. But still undesirable. It obfuscates the meaning of people who are talking about either of the concepts specifically.

I was curious enough to look into some background. "Different but basically the same for practical purposes" seems to be the conclusion.