Bias List

Propositional Fallacies


Affirming a Disjunct

Concludes that one disjunct of a logical disjunction must be false because the other disjunct is true; A or B; A; therefore not B. For example: "Max is a cat or Max is a mammal. Max is a cat. Therefore, Max is not a mammal."


Affirming the Consequent

The antecedent in an indicative conditional is claimed to be true because the consequent is true; if A, then B; B, therefore A. One way to demonstrate the invalidity of this argument form is with a counterexample with true premises but an obviously false conclusion. For example: "If Bill Gates owns Fort Knox, then he is rich. Bill Gates is rich. Therefore, Bill Gates owns Fort Knox."


Appeal to Popular Belief

Concludes that a proposition is true because many or most people believe it: "If many believe so, it is so."


Denying the Antecedent

The consequent in an indicative conditional is claimed to be false because the antecedent is false; if A, then B; not A, therefore not B. For example: "If Queen Elizabeth is an American citizen, then she is a human being. Queen Elizabeth is not an American citizen. Therefore, Queen Elizabeth is not a human being."