**An argument is valid if and only if the following is impossible: all of the premises are true and the conclusion is false. **E.g. the argument below is valid:(1) All men are mortal(2) Socrates is a man(3) So, Socrates is mortalThe argument is valid because it *cannot be the case* that (1) and (2) are both true while (3) is false. The following argument, by contrast, is invalid:(1) Some men have beards(2) Socrates is a man(3) So, Socrates has a beardThis argument is invalid because *it could be the case* that (1) and (2) are both true while (3) is false. After all, it could be the case that Socrates is one of those men who don't have a beard.