Logic : ezyang's blog

Logic

A classical logic fairy tale April 7, 2013

(Selinger) Here is a fairy tale: The evil king calls the poor shepherd and gives him these orders. “You must bring me the philosophers stone, or you have to find a way to turn the philosopher’s stone to gold. If you don’t, your head will be taken off tomorrow!” What can the poor shepherd do to save his life?

Hat tip to Chris for originally telling me a different variant of this story. Unfortunately, this quote from Lectures on the Curry-Howard Isomorphism was the only reference I could find. What should the shepherd do? Is there something a little odd about this story?

An Interactive Tutorial of the Sequent Calculus May 22, 2012

You can view it here: An Interactive Tutorial of the Sequent Calculus. This is the “system in three languages” that I was referring to in this blog post. You can also use the system in a more open ended fashion from this page. Here’s the blurb:

This interactive tutorial will teach you how to use the sequent calculus, a simple set of rules with which you can use to show the truth of statements in first order logic. It is geared towards anyone with some background in writing software for computers, with knowledge of basic boolean logic.
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Many-valued logics and bottom March 11, 2011

I was flipping through An Introduction to Non-Classical Logic by Graham Priest and the section on many-valued logics caught my eye. Many-valued logics are logics with more than the usual two truth values true and false. The (strong) Kleene 3-valued logic, sets up the following truth table with 0, 1 and x (which is thought to be some value that is neither true nor false):

NOT
    1 0
    x x
    0 1

AND
      1 x 0
    1 1 x 0
    x x x 0
    0 0 0 0

OR
      1 x 0
    1 1 1 1
    x 1 x x
    0 1 x 0

IMPLICATION
      1 x 0
    1 1 x 0
    x 1 x x
    0 1 1 1

I’ve always thought many-valued logics were a bit of a “hack” to deal with the self-referentiality paradoxes, but in fact, Kleene invented his logic by thinking about what happened with partial functions where applied with values that they were not defined for: a sort of denotation failure. So it’s not surprising that these truth tables correspond to the parallel-or and and operators predicted by denotational semantics.