We've all probably made easily avoidable errors of logic. Therefore, everyone will want to read an article like this in order to avoid doing so in the future.

There are, of course, several logical problems with the preceding two sentences (we're so predictable), but we'll only mention one: the concluding (second) sentence assumes a premise which probably isn't true, specifically, that we all care about not making logical errors. If you don't care about such things, you are an execrable cretin and may run along and read about homebrewing or tattoos. If, however, you do care, please read on and learn how to be more logical.


When we talk about logic in this article, we aren't talking about symbolic or formal logic. We're talking about the logical rules that govern the making and evaluation of arguments. When we talk about arguments, we mean a premise or series of premises that is or are intended to lead to a conclusion. A "non sequitur" is when such good intentions fail.

"Non sequitur" is Latin for "it does not follow." To say that an argument is a non sequitur is simply to say that the conclusion does not follow from the premises. This term would apply to any argument that has a conclusion that doesn't follow from its premises. It is often used, however, to refer to particular types of arguments that clearly do not follow from their premises and never could.

For example, any argument that takes the following form is a non sequitur:

Step 1: If A then B If I am a goat, I am a hoofed creature. Step 2: B I am a hoofed creature. Step 3: Therefore, A Therefore, I am a goat.

It is clear that this argument does not follow. Even if the premises and conclusion were all true, the conclusion is not a necessary consequence of the premises. This sort of non sequitur is also called "affirming the consequent."

Another common non sequitur is this:

Step 1: If A then B If I am in Rome, I am in Italy. Step 2: Not A I am not in Rome. Step 3: Therefore, not B Therefore, I am not in Italy.

The speaker could be in all kinds of other places in Italy, worrying way less about pickpockets and having a great time. This sort of non sequitur is called "denying the antecedent."