In propositional logic, affirming the consequent (also known as converse error, fallacy of the converse, or confusion of necessity and sufficiency) is a formal fallacy (or an invalid form of argument) that is committed when, in the context of an indicative conditional statement, it is stated that because the consequent is true, therefore the antecedent is true.
It takes on the following form: which may also be phrased as 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.
Two related valid forms of logical argument include modus tollens (denying the consequent) and modus ponens (affirming the antecedent).
[3] Affirming the consequent is the action of taking a true statement
[5][6] Affirming the consequent can also result from overgeneralizing the experience of many statements having true converses.
are equivalent and both true consequences of the statement "August 13 is my birthday" (an abbreviated form of
Of the possible forms of "mixed hypothetical syllogisms," two are valid and two are invalid.
[7] Example 1 One way to demonstrate the invalidity of this argument form is with a counterexample with true premises but an obviously false conclusion.
This argument was featured in Euguene Ionesco's Rhinoceros in a conversation between a Logician and an Old Gentleman.
Example 3 In Catch-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 chaplain's name may be written, but he did not necessarily write it, as the colonel falsely concludes.