Any argument of the following form is invalid: (1) If A then B (2) Not A (3) Therefore, Not B

Examples

1. If you get hit by a car when you are six then you will die young. But you were not hit by a car when you were six. Thus you will not die young. (Of course, you could be hit by a train at age seven, in which case you still die young.)
2. If I am in Calgary then I am in Alberta. I am not in Calgary, thus, I am not in Alberta.

Proof

Show that even though the premises are true, the conclusion may be false. In particular, show that the consequence B may occur even though A does not occur.