Validity

Validity is an idea that is used in everyday language and in logic. In ordinary language it means correct or in the right form.

An argument is valid if it seems appropriate, well-grounded and can be defended. A contract is valid if it is enforceable in law. In other words, it applies legally to a particular situation. A bank-note is valid if it can be exchanged and used for purchase, and a passport is valid if it lets the holder come in and go out of a country. A will is valid if correctly made, and certified by an appropriate court. In all these cases, valid refers to the soundness in some situation. Logical uses of this idea are similar, but phrased much more exactly.

 In logic 
It is nothing to do with ordinary ideas of truth. Truth points outwards to the world. Validity, on the other hand, points inwards to the syllogism.

The conclusion of a valid argument is a logical consequence of its premises. An example of a valid argument is given by the following well-known syllogism:

All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.Michael Frede, Essays in Ancient Philosophy (Minneapolis: University of Minnesota Press, 1987), p. 104

However, this is also a valid syllogism:

All cups are green.
Socrates is a cup.
Therefore, Socrates is green.

And it is not true that Socrates was green.

The following is valid but not sound:

 All animals live on Mars.
 All humans are animals.
 Therefore, all humans live on Mars.

The problem with the argument is that it is not sound.  In order for a deductive argument to be sound, the deduction must be valid and all the premises true.Stephen Priest, The British Empiricists (Oxford; New York: Routledge, 2007), p. 193 

 References 

Category:Logic