Robin
Penultimate Amazing
- Joined
- Apr 29, 2004
- Messages
- 14,971
It is pretty easy to show that modus ponens depends on the axiom of non-contradiction.
Suppose you have
P1 p->q
P2 p
Conclusion q
But any conclusion C can only be a consequence of an argument A if there is no interpretation of A in which C is false. So we add the negation of C to A
P1 p->q
P2 p
P3 ~q
If q can be true and false at the same time then this is a valid interpretation of A in which the conclusion is false and therefore C cannot follow from A.
So modus ponens does depend upon the axiom of non-contradiction.
Suppose you have
P1 p->q
P2 p
Conclusion q
But any conclusion C can only be a consequence of an argument A if there is no interpretation of A in which C is false. So we add the negation of C to A
P1 p->q
P2 p
P3 ~q
If q can be true and false at the same time then this is a valid interpretation of A in which the conclusion is false and therefore C cannot follow from A.
So modus ponens does depend upon the axiom of non-contradiction.