Any argument of the following form is invalid: (1) If A then B (2) B Therefore, A.
- If I am in Calgary, then I am in Alberta. I am in Alberta, thus, I am in Calgary. (Of course, even though the premise is true, I might be in Edmonton, Alberta.)
- If the mill were polluting the river then we would see an increase in fish deaths. And fish deaths have increased. Thus, the mill is polluting the river.