Firmitas

Affirming the consequent

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© 2011-2019 by Zack Smith. All rights reserved.

Affirming the Consequent

This is also known as a Hasty Generalization of a Converse or Converse Error.

If we already know that some proposition P implies some proposition Q, in Affirming the Consequent, the arguer insists or assumes that Q implies P as well.

Its form
  • Proposition P implies Proposition Q.
  • Therefore it is asserted that Q implies P.

Examples

Let P be There is an infection, and Q is There is disease. An arguer claims that because infection P implies disease Q that Q implies P as well. But this does not logically follow. You can have disease due to other causes, in the absense of an infection, such as malnutrition, alcoholism and cancer.

When hungry bears are present, one tends to find dead mooses here and there. However that does not mean the converse is true, that the presence of dead mooses (without additional details) implies the presence of hungry bears. To argue that is to make the Converse Error. Other predators kill mooses as well, for instance human hunters, and mooses can die naturally.

Weaknesses

If claims P and Q were always true or false together, that would be one thing, but in the case of Affirming the Consequent it's only established that P implies Q, yet the arguer insists that Q implying P. You need only point out this error.

Often the arguer wants to believe that Q alone implies P, even though it's not established. In that case, you can provide other causes besides Q that are well known to imply P.