Everyday logic

A Saturday Morning Breakfast Cereal linked to by David Craig and Dan Everett on Facebook:

This could have been framed as a valid inference — but it isn’t here.

A valid inference:

If P is false, I will be sad.
I will not be sad.
Therefore, P is true.

Modus tollens gives us not(P is false), which is equivalent to P is true.

But in the SMBC version, the second premise is modalized, to an expression of a wish or desire, rather than a simple assertion. The most the speaker could conclude is that he wishes that P is true. So what we’re seeing is the triumph of wishful thinking.


One Response to “Everyday logic”

  1. Greg Lee Says:

    The correspondence between a sentence after “if” and the corresponding sentence as an independent clause is not straightforward. The only interpretation I can find for “If p is false, I will be sad” is that the antecedent and consequent refer to future events: p will come to be false, I will come to be sad. Modus ponens is dodgy, and I don’t think modus tollens makes any sense at all.

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