In logic, an argument is said to be *valid* (noun: *validity*) if, and only if, it is the case that, if the premises of the argument are true, the conclusion *must* be true. In other words, a valid argument is one where the premises make the conclusion *have to be* true. There are many other ways to formulate this basic definition: the premises entail the conclusion; it cannot be the case both that the premises are true and the conclusion false; the falsehood of the conclusion entails the falsehood of at least one premise; etc.

Validity is not to be confused with SoundNess; a sound argument is not only valid, its premises are true as well. Not all valid arguments are "valid" in the LooseAndPopularSense? of this word, meaning "good": not all valid arguments (valid, as this term is used in logic) are good, or successful. Here is an example of a valid but very bad argument:

- 1. If Jimbo is wrong, then I'm a monkey's uncle.
- 2. Jimbo is wrong.
- 3. Therefore, I am a monkey's uncle.

This follows one of the most common valid ArgumentForm?s, ModusPonens?: if it's the case that P implies Q, and P is the case, then we can infer that Q. Whenever P implies Q and P is the case, then Q *must* be the case. So, if it *really were true* that Jimbo's wrongness implied my having simian relations (which of course it's not), *and* it were true that Jimbo is wrong (plausible), then it would *have to be* the case that I am a monkey's uncle.