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.

1. AVOID "NON SEQUITURS"

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."

2. DO NOT "BEG THE QUESTION"

We should start out by clarifying something. "Begging the question" is entirely separate from "prompting the question" or "suggesting the question." Many people incorrectly use the term "begging the question" to refer to anything which makes people curious or puts them in mind of a question they'd like answered. That's wrong. Begging the question refers to the practice, in argument, of assuming something which is material to the point you're trying to make. Another way of putting this is that you beg the question when the truth of your conclusion is assumed by the premises that purport to prove it.

For example: "We should not adopt gun control because unacceptable impositions on our liberty cannot be tolerated." If the premise of this argument, i.e. that "unacceptable impositions on our liberty cannot be tolerated," is construed to be relevant to the conclusion about gun control, then the premise must assume that gun control is an unacceptable imposition. If that's the case, then of course we shouldn't adopt it, but that argument begs the question of whether or not gun control is a good thing.

3. IGNORE THE IDENTITY OF THE ARGUER

The identity of a speaker or writer is irrelevant to the validity of his or her argument. It simply does not matter who uttered an argument. All that matters is whether the premises lead to the conclusion. There are only two ways to criticize an argument: (1) One or more of the premises is or are false, or (2) the premises do not lead to the conclusion. This is elementary logic, but it eludes many people.

You may have heard people discuss, with disfavor, "ad hominem" arguments. Ad hominem is Latin for "to the man." An ad hominem argument tries to attack characteristics of a person which are irrelevant to the issue. When you evaluate an argument with reference to the speaker, you are making an ad hominem response. Again, this is wrong; you must respond to the argument, not "to the man."

People believe that the identity of the speaker is important because they think they must be watchful for bias or hypocrisy. This is incorrect, in the context of evaluating arguments. It is true that bias can cause people to be dishonest, and you should be concerned about bias when you evaluate whether or not to believe a person's reports of fact. However, this is still irrelevant to an argument, because the premises are either true or untrue on their own merits. If you decide not to believe a biased speaker, it essentially means that you refuse to evaluate an argument that contains the questionable statement as a premise. It is no criticism of the argument that you are skeptical of the reliability of its premises. In order to refute the argument, you need to prove that one of the premises is false.

Hypocrisy is even less relevant to arguments. For example:

    Speaker 1: It is wrong to steal.

    Speaker 2: But you stole something yesterday!

Speaker 2's comment is not responsive to the remark of Speaker 1. Speaker 1 was making a statement about what is wrong, and Speaker 2 changed the subject by talking about Speaker 1's behavior. It is the same when someone presents you with an argument and you begin to discuss his or her identity. You are no longer discussing the argument; you have changed the subject.

Just look at arguments based on their merits. If you don't trust a source that is reporting as fact one of the premises, it is okay to say that you don't think you can evaluate the argument until you have verified one of the premises. What is not okay is this sort of thing:

    Speaker A: Marijuana should be decriminalized because the criminal law should only forbid citizens from committing acts which harm persons or the property of others.

    Speaker B: You're just saying that because you like to smoke it so much.

What is relevant to an evaluation of A's argument is whether or not his single premise, about what the criminal law should forbid, is true. Whether or not he likes to smoke the stuff doesn't enter into it. Always ignore the identity of the source when you evaluate an argument.

4. DO NOT USE ANECDOTAL EVIDENCE TO PROVE A POINT

Anecdotal evidence proves nothing. Well, perhaps we're being too hasty. If we assume that the anecdotal evidence we are hearing is true, we could use it to disprove a universal claim. For example, if someone claims that all midgets are blind, you could disprove the claim by reporting that you have a cousin who is a sighted midget. If we assume that you are speaking the truth, then it cannot be true that all midgets are blind. What we could not do is extrapolate from the happy case of your cousin to a statement about the sightedness of midgets in general.

In most contexts, anecdotal evidence is worse than useless, because it is misleading. If there are fifty people in a room, and ten of them have little stories to share about the warmth and compassion of Texans, should we take their stories as evidence for the proposition that most Texans are warm and compassionate? Of course we shouldn't; we haven't heard about even a tiny fraction of the Texans in the world. In this case, we've only heard about a portion of the behavior of Texans met by ten people. Those Texans could have been vicious at other times (see, e.g., the Alamo). In any event, the time spent listening to the ten people's stories is wasted time, if the purpose of the discussion is to establish some general factual proposition about Texans.

5. DO NOT COMMIT THE "POST HOC, ERGO PROPTER HOC" FALLACY

"Post hoc, ergo propter hoc" is Latin for "after this, therefore because of this." It is used to refer to the logical fallacy of assuming that because one event follows another it was also caused by it. For example: "I thought of my brother, and then, like, two seconds later he phoned me. He must have felt my aura and responded to me." Or, less fancifully, but equally flawed: "The rooster cock ‘a doodled and the sun rose. Therefore, the rooster's sound must have caused the sun to rise."

Sometimes an event happens after another event coincidentally, and the two have nothing to do with each other. The mere fact of proximity in time should not be taken as sufficient proof of a cause-and-effect relationship, because it ain't.

At other times, a number of partial causes lead to an effect, but one is selected as the sole cause. This is called "oversimplification of the cause," and it is also flawed thinking. We can't establish what is actually the case solely by reference to when things happen. So long as you don't fall into thinking that you can thusly establish things, you won't commit the post hoc, ergo propter hoc fallacy.

6. DO NOT ARGUE BY ANALOGY

Our final suggestion is our most controversial one. Many people are big fans of arguing by analogy; the legal profession would be nowhere without it. Arguing by analogy can be done in one bold sweeping statement, for example, "Just as it was wrong to deny women the vote, it is also wrong to deny the vote to children." It can also be a lengthy, involved process, where two speakers agree that a rule or predicate applies to one state of affairs, and then argue over whether another state of affairs is enough like the first state of affairs for the same rule or predicate to apply to it. A time-consuming but rewarding task, right? Wrong.

The problem? This is a waste of time, because analogies don't prove anything. An analogy can be useful to illustrate a point or explain something to someone who is unfamiliar with a concept. For example: "You are familiar with broccoli, right? Well cauliflower is like broccoli except that it is white" Analogies can also make an argument accessible or understandable to someone who is otherwise unwilling to be open-minded about it. For example: "Those who are unwilling to accept the possibility that animals deserve moral consideration should consider that much the same thing used to be said about racial minorities."

You should notice that this example is not an argument. It is an attempt to get people who are simply closed-minded to look at the argument (the one the analogy is trying to support, that is) more neutrally. Some people believe, however, that examples like this are arguments, and that we should treat them as such. They are wrong, and the reason for this is very simple: they are trying to argue about one thing by talking about something else. This is pointless. If you think that the line of argument that led you to believe something about X might also apply to Y, don't talk about X. Apply the reasoning to Y and see if it works. Leave the analogies to the lawyers and storytellers.

And here we'll end abruptly because we do not care about style--only logic. (If you did not notice a logical problem with this last sentence, consider reading this article again -- or check out another good site on this topic.)