Corresponding conditional (logic)

From Wikipedia, the free encyclopedia

This article or section needs to be wikified to meet Wikipedia's quality standards. Please help improve this article with relevant internal links. (June 2007)

In logic a corresponding conditional is a statement whose principal connective is the material implication symbol, and whose antecedent is the conjunction of the premises or an argument and whose consequent is the conclusion of that argument. An argument is valid if and only of its corresponding conditional is a necessary truth.

[edit] example

If an argument A were of the form

Either P or Q

Not P

Therefore Q

Then its corresponding conditional C would be:

((PQ) & ¬P) Q

and A is valid if and only if C is a necessary truth.

If C is a necessary truth then ¬C entails The False. It follows that any argument A is valid if and only if the denial of its corresponding conditional leads to a contradiction.

In the above example, starting with denial of the corresponding conditional:

Premises	Line		Comment
1	1	~(((PvQ) & ~P)⊃ Q)	premis
1	2	((PvQ) & ~P)& ~Q)	1, definition of ⊃
1	3	((P&~P) v (Q&~Q)) & Q	2, de morgan
1	4	((P&~P)&Q) v ((Q&~Q)&Q))	3, de morgan
1	5	(P&~P) v (Q&~Q)	4, weakening
	6	(~(((PvQ) & ~P)⊃ Q) )⊃(P&~P) v (Q&~Q)	1, 5 conditionalising
	7	~((P&~P) v (Q&~Q))	axiom
	8	(((PvQ) & ~P)⊃ Q)	6,7 modus tonens

We could of course have, instead, created a truth table for((PvQ) & ~P)-> Q) to show that it is a tautology.

[edit] External links

• corresponding conditional