Affirming the Consequent

This lecture introduces the idea of a formal logical fallacy—that is, an argument that does not follow from its premises. Affirming the consequent is one such fallacy.

Recall that modus ponens states that the premises

P => Q
P

allow us to conclude Q.

Affirming the consequent attempts to make a similar argument using the premises

P => Q
Q

to conclude P. But these two premises do not allow us to conclude anything about Q; Q may or may not be true.

We call it “affirming the consequent” because Q is the consequent in the implication and the second premise affirms it.

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