My seminar keeps marking arguments as valid but not sound, and I lose the distinction every time I think I have it. Both seem to mean the argument is good.
Is there a case where calling something valid is actually a criticism?
Accepted Answer
Validity is about the shape. Soundness is about the shape and the contents.
An argument is valid if the conclusion cannot be false while the premises are true. That is a claim about structure and says nothing about whether the premises are actually true.
It is sound if it is valid and the premises really are true.
So yes, valid can be a criticism, and this is the standard example:
All cats are made of glass. Fluffy is a cat. Therefore Fluffy is made of glass.
Perfectly valid. The conclusion genuinely follows. It is also nonsense, because the first premise is false. Valid, not sound.
Saying "but that is valid" about an argument like that is pointing out that the only thing going for it is its wiring.
One trap here that catches people right after the definition lands.
Validity is not affected by whether the conclusion is true. You can have a valid argument with a true conclusion and false premises, and an invalid argument with a true conclusion.
All fish swim. Dolphins swim. Therefore dolphins are fish. The conclusion is false and the form is broken. But swap in "Sharks swim, therefore sharks are fish" and you get a true conclusion from the same broken form.
Checking the conclusion tells you nothing about validity. Only the form does.
A short way to hold it: validity is a promise about the inference, soundness is that promise plus the premises being paid up.
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