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Affirming the Consequent

Any argument of the following form is invalid: (1) If A then B (2) B Therefore, A.


  1. 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.)
  2. 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.


Show that even though the premises are true, the conclusion could be false. In general, show that B might be a consequence of something other than A. For example, the fish deaths might be caused by pesticide run-off, and not the mill.