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Affirming the consequent, sometimes called converse error, fallacy of the converse, or confusion of necessity and sufficiency, is a
formal fallacy In philosophy, a formal fallacy, deductive fallacy, logical fallacy or non sequitur (; Latin for " tdoes not follow") is a pattern of reasoning rendered invalid by a flaw in its logical structure that can neatly be expressed in a standard logic sy ...
of taking a true conditional statement (e.g., "If the lamp were broken, then the room would be dark"), and invalidly inferring its converse ("The room is dark, so the lamp is broken"), even though that statement may not be true. This arises when a consequent ("the room would be dark") has other possible antecedents (for example, "the lamp is in working order, but is switched off" or "there is no lamp in the room"). Converse errors are common in everyday thinking and communication and can result from, among other causes, communication issues, misconceptions about logic, and failure to consider other causes. The opposite statement, denying the consequent, ''is'' a valid form of argument ( modus tollens).

# Formal description

Affirming the consequent is the action of taking a true statement $P \to Q$ and invalidly concluding its converse $Q \to P$. The name ''affirming the
consequent A consequent is the second half of a hypothetical proposition. In the standard form of such a proposition, it is the part that follows "then". In an implication, if ''P'' implies ''Q'', then ''P'' is called the antecedent and ''Q'' is called ...
'' derives from using the consequent, ''Q'', of $P \to Q$, to conclude the antecedent ''P''. This fallacy can be summarized formally as $\left(P \to Q, Q\right)\to P$ or, alternatively, $\frac$. The root cause of such a logical error is sometimes failure to realize that just because ''P'' is a ''possible'' condition for ''Q'', ''P'' may not be the ''only'' condition for ''Q'', i.e. ''Q'' may follow from another condition as well. Affirming the consequent can also result from overgeneralizing the experience of many statements ''having'' true converses. If ''P'' and ''Q'' are "equivalent" statements, i.e. $P \leftrightarrow Q$, it ''is'' possible to infer ''P'' under the condition ''Q''. For example, the statements "It is August 13, so it is my birthday" $P \to Q$ and "It is my birthday, so it is August 13" $Q \to P$ are equivalent and both true consequences of the statement "August 13 is my birthday" (an abbreviated form of $P \leftrightarrow Q$). Of the possible forms of "mixed hypothetical syllogisms," two are valid and two are invalid. Affirming the antecedent ( modus ponens) and denying the consequent ( modus tollens) are valid. Affirming the consequent and denying the antecedent are invalid.Kelley, David (1998), ''The Art of Reasoning'' (3rd edition). Norton, pp. 290–94.

Example 1 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 someone lives in
San Diego San Diego ( , ; ) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United States ...
, then they live in
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
. :Joe lives in California. :Therefore, Joe lives in San Diego. There are many ways to live in California without living in San Diego, as long as they live in a Californian place other than San Diego. However, one can affirm with certainty that "if someone does not live in California" (''non-Q''), then "this person does not live in San Diego" (''non-P''). This is the contrapositive of the first statement, and it must be true if and only if the original statement is true. Example 2 Here is another useful, obviously fallacious example. :If an animal is a dog, then it has four legs. :My cat has four legs. :Therefore, my cat is a dog. Here, it is immediately intuitive that any number of other antecedents ("If an animal is a deer...", "If an animal is an elephant...", "If an animal is a moose...", ''etc.'') can give rise to the consequent ("then it has four legs"), and that it is preposterous to suppose that having four legs ''must'' imply that the animal is a dog and nothing else. This is useful as a teaching example since most people can immediately recognize that the conclusion reached must be wrong (intuitively, a cat cannot be a dog), and that the method by which it was reached must therefore be fallacious. Example 3 Arguments of the same form can sometimes seem superficially convincing, as in the following example: :If Brian had been thrown off the top of the
Eiffel Tower The Eiffel Tower ( ; french: links=yes, tour Eiffel ) is a wrought-iron lattice tower on the Champ de Mars in Paris, France. It is named after the engineer Gustave Eiffel, whose company designed and built the tower. Locally nicknamed "' ...
, then he would be dead. :Brian is dead. :Therefore, Brian was thrown off the top of the Eiffel Tower. Being thrown off the top of the Eiffel Tower is not the ''only'' cause of death, since there exist numerous different causes of death. Example 4 In '' Catch-22'', the chaplain is interrogated for supposedly being "Washington Irving"/"Irving Washington", who has been blocking out large portions of soldiers' letters home. The colonel has found such a letter, but with the Chaplain's name signed. :"You can read, though, can't you?" the colonel persevered sarcastically. "The author signed his name." :"That's my name there." :"Then you wrote it. Q.E.D." ''P'' in this case is 'The chaplain signs his own name', and ''Q'' 'The chaplain's name is written'. The chaplain's name may be written, but he did not necessarily write it, as the colonel falsely concludes.''