
Affirming the Consequent
Definition:
Any argument of the following form is invalid:
If A then B
B
Therefore, A
Examples:
 If I am in Calgary, then I am in Alberta. I am in Alberta,
thus, I am in Calgary. (Of course, even though the premises
are 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.
Proof:
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 runoff, and not the mill.
References
Barker: 69, Cedarblom and Paulsen: 24, Copi and Cohen: 241
26 May 1995 / 06 January 1996
