Review: Soundness

DEFINED: A deductive argument is sound when:
(1) the argument is valid; and
(2) all the premises are true.

WHAT IT MEANS: This is the gold standard—as good as it gets for a deductive argument. Sound deductive arguments are PERFECT. From the start, a sound argument has to be valid. But on top of it, each premise must also be true.

So to test for soundness, we first do our validity test. We do the imagine-a-world test. If the arg fails this test, then it's invalid, and automatically unsound.

If it passes the validity test, then we need to check the actual truth of the premises. So we forget about the imaginary world, and come back to the real world. Are all of the premises actually true in the real world, or is one or more false or questionable (opinionated)? If they're all true, then the argument is sound. If one or more is false or questionable, then the argument is unsound.

An argument is unsound if it is not sound. (duh) But remember, it’s easy for an argument to be unsound. Only one of many things has to go wrong for an argument to be unsound. One false premise, and it’s unsound. One false move in an argument that makes it invalid, and it’s unsound. Any deductive argument that isn't PERFECT is unsound.