1 definition by John Joseph O'Hara
A property of an argument in logic that preserves the truth, in which there is a neccessary agreement between the premises (assuming that they are true) and the conclusion; that one must undoubtedly arrive at the conclusion. It is not possible for the premises to be true, and the conclusion false. Note that validity is not the same as soundness, which is a valid argument that has all true premises.
Valid:
If I am in Paris, then I am in France; I'm in Paris, therefore I am in France. If I am not in France, then I am not in Paris; oh look, I'm in New York; therefore I must not be in France.
Invalid:
If I am in France, then I am in Paris; Woah! I'm in France, I must be in Paris...BUZZ! I'm actually in Bordeaux.
Either 2 plus 2 equals 22, or Santa Clause is real; but 2 plus 2 does not equal 22; so Santa Clause is real.
Unsound but Valid:
All mammals are cats (false, but assuming it is true); All cats are animals; So all mammals are animals.
If I am in Paris, then I am in France; I'm in Paris, therefore I am in France. If I am not in France, then I am not in Paris; oh look, I'm in New York; therefore I must not be in France.
Invalid:
If I am in France, then I am in Paris; Woah! I'm in France, I must be in Paris...BUZZ! I'm actually in Bordeaux.
Either 2 plus 2 equals 22, or Santa Clause is real; but 2 plus 2 does not equal 22; so Santa Clause is real.
Unsound but Valid:
All mammals are cats (false, but assuming it is true); All cats are animals; So all mammals are animals.
by John Joseph O'Hara April 10, 2006