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Comment on Is "If A then B" equal to "B if and only if A"?

That’s not equivalent either. “if not b, then not a” works if it’s a sequence but doesn’t work for options in which multiple inputs can lead to the same output. If you get pizza every Tuesday and Friday, then answering “what’s for lunch” with “if Tuesday, then pizza” and “if Friday, then pizza” doesn’t let it work in reverse. “what day is it” can’t be answered with “if pizza lunch, then Tuesday”

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Boinkage@lemmy.world ⁨1⁩ ⁨year⁩ ago
Ya wrong.

If Tuesday, then pizza. And, if Friday, then pizza.

The contrapositive : if not pizza, then not Tuesday and not Friday.

What day is it? We’re not having pizza. So it’s not Tuesday or Friday.

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- XeroxCool@lemmy.world ⁨1⁩ ⁨year⁩ ago
  That’s a key detail I now see
  
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pruwybn@discuss.tchncs.de ⁨1⁩ ⁨year⁩ ago
You left out the “not” part - “If not pizza lunch, then not Tuesday” does indeed work.

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kogasa@programming.dev ⁨1⁩ ⁨year⁩ ago
Using standard definitions from propositional logic they are equivalent.

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