Definition
An argument is sound when it is valid and all of its premises are in fact true. Soundness is the gold standard for an argument that should actually convince us: the reasoning is watertight and the starting points are correct, so the conclusion is guaranteed to be true.
Validity alone is not enough. A valid argument can lead from false premises to a false conclusion while remaining perfectly valid. Soundness adds the missing ingredient — factual truth — and only then does the conclusion inherit a genuine claim on our belief.
Why it matters
How it works
Evaluating soundness is a two-stage task. First, check the form: is the argument valid, so that true premises would force a true conclusion? This is logic's job. Second, check the content: are the premises actually true? This is the job of observation, science, or expert testimony — not of logic itself.
Priest stresses that logic delivers only the first half. It can certify that an inference is valid, but it cannot tell you whether "All swans are white" is true. Soundness therefore marks the boundary between formal reasoning and empirical inquiry: logic guards the link, the world supplies the facts.