Why does justification require necessary truth?Plato set West philosophy on a long search for a theory of justification which would show that to be genuine knowledge it is not sufficient merely to hold a true belief, but one must also be in a position to justify that belief as necessarily true.
The basis of this move lies in Plato's implicit acceptance of a "correspondence theory of truth." (This is a contemporary label for this view of truth, not Plato's own.) According to this view "truth" consists in a relation of "correspondence" between what is said or believed to be the case and what, in reality is the case. Plato expresses this by concluding that "the object of knowledge must be real."
Now given these commitments, we can prove the conclusion that to be known a belief must be justified as necessarily true by the technique of reductio ad absurdum (i.e. "reduction to a contradiction). To do this we show that denying the conclusion to be proved leads to a contradiction, so it is logically impossible to deny that conclusion. Applying this technique we get this argument:
Suppose we were to consider the possibility that S knows P, but P is not true, or in other words, S knows what is false. Then, by the correspondence theory of truth P states what not the case in reality. But if it is not the case in reality then what it is alleged to be knowledge of, its object, is not real. But Plato has already defined knowledge in contrast to belief as requiring a real object. Therefore the attempt to "know" what is false, is self conttradictory. Therefore it is logically impossible to know what is false, or put in the language of logic,The "justification condition" thus becomes in effect the demand that to know a proposition requires being able to show that proposition is necessarily true.If "S knows P," then P must be necessarily true.
The word "proof" is correctly used in its logical sense only to refer to such a demonstration of the necessary truth of a proposition. Thus P is justified if P can be "proved."