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.

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*.

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

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

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).