Affirming the consequent

If A then B.  B, therefore A

An argument following this logic is invalid:
If A then B
therefore A.

If I’m in Spain, I’m in Europe. Since I’m in Europe, I’m in Spain (it is obvious that even if the premises are correct, i could be in France).
If the factory contaminated the river, there would be an increase in fish deaths. Fish deaths increased, hence the factory is contaminating the river.

To refute it:

Prove the conclusion could be wrong even if the premises are right. In general, prove that B could happen as a consequence of a different possibility than A. For instance, the fish deaths could be due to uncontrolled pesticides and not necessarily because of the factory’s activity.

Whenever the source is not referenced, both definitions and examples have been extracted from a translation of Jaime Wilson based on Stephen’s Guide to the Logical Fallacies. Copyright 1995-1998 Stephen Downes. Brandon, Manitoba, Canada.

These texts have been modified by Miguel A. Lerma and now by us to adapt them -and those taken from Wikipedia- to our format.