Inpropositional logic,affirming the consequent (also known asconverse error,fallacy of the converse, orconfusion ofnecessity and sufficiency) is aformal fallacy (or aninvalid form of argument) that is committed when, in the context of anindicative conditional statement, it is stated that because theconsequent is true, therefore theantecedent is true. It takes on the following form:

IfP, thenQ.

Q.

Therefore,P.

which may also be phrased as

${\displaystyle P\to Q}$ (P implies Q)

${\displaystyle \therefore Q\to P}$ (therefore, Q implies P)

For example, it may be true that a broken lamp would cause a room to become dark. It is not true, however, that a dark room implies the presence of a broken lamp. There may be no lamp (or any light source). The lamp may also be off. In other words, the consequent (a dark room) can have other antecedents (no lamp, off-lamp), and so can still be true even if the stated antecedent is not.^[1]

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.^[2]

A related fallacy isdenying the antecedent. Two relatedvalid forms of logical argument includemodus tollens (denying the consequent) andmodus ponens (affirming the antecedent).^[3]

Affirming the consequent is the action of taking a true statement${\displaystyle P\to Q}$ and invalidly concluding its converse${\displaystyle Q\to P}$. The nameaffirming theconsequent derives from using the consequent,Q, of${\displaystyle P\to Q}$, to conclude the antecedentP. This fallacy can be summarized formally as${\displaystyle (P\to Q,Q)\to P}$ or, alternatively,${\displaystyle \frac{P\to Q,Q}{\therefore P}}$.^[4]The root cause of such a logical error is sometimes failure to realize that just becauseP is apossible condition forQ,P may not be theonly condition forQ, i.e.Q may follow from another condition as well.^[5][6]

Affirming the consequent can also result from overgeneralizing the experience of many statementshaving true converses. IfP andQ are "equivalent" statements, i.e.${\displaystyle P\leftrightarrow Q}$, itis possible to inferP under the conditionQ. For example, the statements "It is August 13, so it is my birthday"${\displaystyle P\to Q}$ and "It is my birthday, so it is August 13"${\displaystyle Q\to P}$ are equivalent and both true consequences of the statement "August 13 is my birthday" (an abbreviated form of${\displaystyle P\leftrightarrow Q}$).

Of the possible forms of "mixedhypothetical syllogisms," two are valid and two are invalid. Affirming the antecedent (modus ponens) and denying the consequent (modus tollens) are valid. Affirming the consequent anddenying the antecedent are invalid.^[7]

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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 inSan Diego, then they live inCalifornia.

Joe lives in California.

Therefore, Joe lives in San Diego.

There are many places to live in California other than San Diego. On the other hand, 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 thecontrapositive of the first statement, and it must be true if and only if the original statement is true.

Example 2

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 legsmust 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. This argument was featured inEuguene Ionesco's Rhinoceros in a conversation between a Logician and an Old Gentleman.

Example 3

InCatch-22,^[8] 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', andQ '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.^[8]

• Formal fallacies

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