Affirming the consequent

In propositional logic, affirming the consequent is a formal fallacy. It occurs when someone takes a true conditional statement and incorrectly infers its converse. This means assuming that if "If A then B" is true, then "If B then A" must also be true, which is not necessarily correct.

This fallacy assumes that an if...then... statement is reversible. Specifically, it mistakenly believes that given "If A then B," it can also be true that "If B then A." In this context, B (the 'then' part) is called the 'consequent,' and A (the 'if' part) is called the 'antecedent.'

Examples:

1. I am in London, England. I am in England; therefore, I am in London.
2. If you are cheating on me, you will be out of the house a lot. You are out of the house a lot, so you must be cheating on me.

This arises when the consequent has other possible antecedents. 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 called modus tollens and is a valid form of argument.