What is a fallacy?

A fallacy is best defined as "a failure in reasoning that renders an argument invalid". The most common example would be the so-called "Politician's Syllogism": Cats have 4 legs, my dog has 4 legs, therefore my dog is a cat. Fallacies do not necessarily have to be so silly (although in religious debates, you will find they very often are), but all of them render an argument invalid.

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