**Affirming the consequent**, sometimes called

**converse error**,

**fallacy of the converse** or

**confusion of necessity and sufficiency**, is a formal fallacy of inferring the converse from the original statement. The corresponding argument has the general form:

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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 of inferring the converse from the original statement. The corresponding argument has the general form:

- If
*P*, then *Q*.
*Q*.
- 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).

To put it differently, if *P* implies *Q*, the **only** inference that can be made is *non-Q* implies *non-P*. (*Non-P* and *non-Q* designate the opposite propositions to *P* and *Q*.) This is known as logical contraposition. Symbolically:

$(P\to Q)\leftrightarrow (\neg Q\to \neg P)$

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

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