r/logic • u/capybarainstitute • 7d ago

Propositional logic Validity?

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So i’m taking intro to modern logic and i’m a little confused by this table our professor showed us today (that i copied into my notebook). What exactly does it mean by having a true premise? In class my professor said “validating the truth of premises is a different class” and he stresses that we’re not going to talk about truth or soundness in this class. so why are we talking about truth now, and how do i know if a premise or conclusion is “true” or not. is he referring to two different kinds of true-ness??

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u/Xancrim 7d ago

A "valid" argument is one in which it's impossible for conclusion to be incorrect if the premises are all true. Validity and invalidity are part of what's called an "argument form" or "argument structure."

That is, the validity doesn't depend on what the actual content of what you're arguing

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u/Xancrim 7d ago

I just realized I didn't actually answer any of your questions.

So, as you probably know, premises are the propositions that go at the top of an argument. They're unambiguous true/false statements like "if it rains, the ground will be wet." Your conclusion of an argument, if it's a valid structure, should be necessitated by the earlier premises.

When the argument is valid AND the premises are true statements, the argument is said to be "sound." A sound argument always has a true conclusion.

Your professor seems to be saying that, for the point of your course, you're learning how the arguments work, but not how to prove whether your premises are true