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The pattern of argument of the form 'If P then Q; Q; therefore P' is known as affirming the consequent (meaning affirming what comes after, in this case Q). This argument pattern is a fallacy and is invalid. An invalid argument has a flawed logical format so even if it's premises were true, they would not lead to the conclusion - in this case P.
Affirming the consequent is in contrast to the valid argument form called modus ponens. This deductive pattern is represented by the form, ' If P then Q; P; therefore Q'. In this instance, if the premises were true, they would necessarily lead to the conclusion, in this case, Q.
This material has been developed independently of the International Baccalaureate, which in no way endorses it.
© Austhink 2013. Rationale Exercises version 0.1, Jan-13
