Affirming the consequent

Affirming the consequent, sometimes called converse error, is a formal fallacy, committed by reasoning in the form:

1. If P, then Q.
2. Q.
3. Therefore, P.

An argument of this form is invalid, i.e., the conclusion can be false even when statements 1 and 2 are true. Since P was never asserted as the only sufficient condition for Q, other factors could account for Q (while P was false).

The name affirming the consequent derives from the premise Q, which affirms the "then" clause of the conditional premise.

Examples

One way to demonstrate the invalidity of this argument form is with a counterexample with true premises but an obviously false conclusion. For example:

If Bill Gates owns Fort Knox, then he is rich.

Bill Gates is rich.

Therefore, Bill Gates owns Fort Knox.

Owning Fort Knox is not the only way to be rich. There are any number of other ways to be rich.

Arguments of the same form can sometimes seem superficially convincing, as in the following example:

If I have the flu, then I have a sore throat.

I have a sore throat.

Therefore, I have the flu.

But having the flu is not the only cause of a sore throat since many illnesses cause sore throat, such as the common cold or strep throat.

The following is a more subtle version of the fallacy embedded into conversation.

A: All Republicans are against gun control.

B: That's not true. My uncle's against gun control and he's not a Republican.

B attempts to falsify A's conditional statement ("if Republican then against gun control") by providing evidence he believes would contradict its implication. However, B's example of his uncle does not contradict A's statement, which says nothing about non-Republicans. What would be needed to disprove A's assertion are examples of Republicans who support gun control.

See also

• Modus ponens
• Modus tollens
• Post hoc ergo propter hoc
• Denying the antecedent
• Fallacy of the undistributed middle
• Inference to the best explanation
• ELIZA effect

References

This logic-related article is a stub. You can help Wikipedia by expanding it.v · Categories: Logic stubs Propositional fallacies Wikimedia Foundation. 2010. Игры ⚽ Поможем сделать НИР Anatomy Andrei Tarkovsky Look at other dictionaries: affirming the consequent — noun A formal fallacy, committed by reasoning in the form:<p>If P, then Q.<p>Q.<p>Therefore, P …   Wiktionary affirming the consequent — Arguing invalidly that from if p then q and q, it follows that p ; e.g. arguing from ‘if she is studying mathematics then she is not studying English’ and ‘she is not studying English’ to ‘she is studying mathematics’ …   Philosophy dictionary fallacy of the consequent — the logical fallacy of affirming the consequent : affirmation of the consequent …   Useful english dictionary The Church —     The Church     † Catholic Encyclopedia ► The Church     The term church (Anglo Saxon, cirice, circe; Modern German, Kirche; Sw., Kyrka) is the name employed in the Teutonic languages to render the Greek ekklesia (ecclesia), the term by which… …   Catholic encyclopedia Fallacy of the undistributed middle — The fallacy of the undistributed middle is a logical fallacy that is committed when the middle term in a categorical syllogism isn t distributed. It is thus a syllogistic fallacy. Pattern The fallacy of the undistributed middle takes the… …   Wikipedia Denying the antecedent — Denying the antecedent, sometimes also called inverse error, is a formal fallacy, committed by reasoning in the form: If P, then Q. Not P. Therefore, not Q. Arguments of this form are invalid. Informally, this means that arguments of this form do …   Wikipedia The Blessed Trinity —     The Blessed Trinity     † Catholic Encyclopedia ► The Blessed Trinity     This article is divided as follows:          I. Dogma of the Trinity;     II. Proof of the Doctrine from Scripture;     III. Proof of the Doctrine from Tradition;… …   Catholic encyclopedia Begging the question — Bust of Aristotle, whose Prior Analytics contained an early discussion of this fallacy. Begging the question (or petitio principii, assuming the initial point ) is a type of logical fallacy in which the proposition to be proven is assumed… …   Wikipedia History of the British Labour Party — This is about the history of the British Labour Party. For information about the wider history of British socialism see History of socialism in Great Britain. For more detailed information about the present Labour government see Current Labour… …   Wikipedia Outline of logic — The following outline is provided as an overview of and topical guide to logic: Logic – formal science of using reason, considered a branch of both philosophy and mathematics. Logic investigates and classifies the structure of statements and… …   Wikipedia 18+ © Academic, 2000-2024 Contact us: Technical Support, Advertising Dictionaries export, created on PHP, Joomla, Drupal, WordPress, MODx. Mark and share Search through all dictionaries Translate… Search Internet Share the article and excerpts Direct link … Do a right-click on the link above and select “Copy Link”