Definition: Any argument of the following form is invalid:
If p then q Not p Therefore, Not q.
Examples:
 If you were to get hit by a car when you are six, then you would 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.)
 If I am in Minneapolis then I am in Minnesota.
I am not in Minneapolis. Therefore, I am not in Minnesota.
(No, I am in St. Paul, Minnesota.)
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.
References Barker: 69, Cedarblom and Paulsen: 26, Copi and Cohen: 241
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