Perhaps the most common of fallacies occurs when evidence is insufficient. We call this a hasty generalization, meaning that there is not enough evidence to support a particular conclusion. For instance: “Smoking isn’t bad for you; my great-aunt smoked a pack a day and lived to be ninety.” It could be that the story of the speaker’s aunt is true, but this single anecdote does not provide enough evidence to discredit the results of years of medical research.

Another fallacy resulting from insufficient evidence is circular reasoning. Circular reasoning involves repeating the claim as a way to provide evidence, resulting in no evidence at all. For instance, a student who asserts, “You can’t give me a C; I’m an A student” is guilty of circular reasoning; that is, the “evidence” that she should get an A is that she is an A student. The so-called evidence is insufficient because it is a mere repetition of the claim. You can frequently spot circular reasoning in advertising. For instance: “Buy this shampoo because it’s the best shampoo!” or “Shop at this store because it’s a shopper’s paradise.”

We will discuss other common logical fallacies as we examine specific types of evidence.