The Formal Fallacies
A formal fallacy is an error in reasoning which is inherent in the structure of the argument itself. There are two formal fallacies, which are the “evil twins” of modus ponens and modus tollens. The first is called Denying the Antecedent, which we will call “DA” for short. Modus tollens is a valid argument because it denies what’s called the consequent i.e. the Q in “if P then Q”. DA is the invalid evil twin of modus tollens because it denies the antecedent i.e the P in “if P then Q”.