ValidIty

In logic, an argument is said to be ''valid'' (noun: ''validity'') iff it is the case that, if the premises of the argument are true, the conclusion ''must'' 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.