In classical two-valued logic, an argument is said to have

**validity**or to be

**valid**if, and only if, it is the case that, if the premises of the argument are true, then the conclusion

*must*be true. In other words, a valid argument is one where the premises

*make*the conclusion 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.

A close examination of the definition of 'valid' should make a few things clear about validity. The definition says neither that the premises have to be true nor that that the conclusion has to be true. Validity is a conditional notion: what it says is that *if* the premises happen to be true, *then* the conclusion has to be true. As far as validity is concerned the premises might be completely and obviously false. Consider an example of a valid argument:

- All dogs have eight legs.
- The President is a dog.
*Therefore*, the President has eight legs.

*true*that all dogs had eight legs; and suppose, just suppose, that the President really were a dog; well, in that absurd imaginary world, the President would have to have eight legs. The conclusion

*has*to be true, if the premises are true. So the argument is valid, even though it has false premises, not to mention a false conclusion.

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 loose and popular sense of this word, meaning 'good': not all valid arguments (valid, as this term is used in logic) are good, or successful, as the above example should show.

*Argument form* is what makes an argument valid. But a valid argument is one where, if the premises are true, then the conclusion must be true (and here is a way to put it more briefly: the premises make the conclusion *necessary*). Now put these two propositions together and draw a conclusion:

- Form makes an argument valid.
- If an argument is valid, then the premises make the conclusion
*necessary*. - Form makes an argument such that the premises make the conclusion
*necessary*.

*just*

*by*

*looking*

*at*

*the*

*form*

*of*

*the*

*argument*. That is why argument form is so important. Look, for example, at the following argument form. In fact,

*any*argument that follows this form is valid. You can see that just by reading it:

- All S is P.
*a*is S.*Therefore*,*a*is P.

- All dogs are canines.
- Fido is a dog.
*Therefore*, Fido is a canine.

*validity*means the

*legal existence of a norm*. A norm that is authorized by another norm is said to exist in that legal system, i.e., to be

*valid*.

In psychometrics, a valid measure is one that measures what it is supposed to measure. For example, a valid measure of mathematical problem-solving measures mathematical ability rather than the verbal ability necessary to understand complicated statements of mathematical problems. See Validity (psychometric).