Avoiding Informal [Logical] Fallacies

Informal fallacies are instances of murky reasoning that can cloud an argument and lead to unsound conclusions. Because they can crop up unintentionally in anyone's writing, and because advertisers and hucksters often use them intentionally to deceive, it is a good idea to learn to recognize the more common fallacies.

Post hoc, ergo propter hoc ("after this, therefore because of this"). This fallacy involves mistaking sequence for cause. Just because one event happens before another event doesn't mean the first event caused the second. The connection may be coincidental, or some unknown third event may have caused both of these events.

Example: For years I suffered from agonizing abdominal itching. Then I tried Jones pills. Almost overnight my abdominal itching ceased. Jones pills work wonders.

Hasty generalization. Closely related to the post hoc fallacy is the hasty generalization, which refers to claims based on insufficient or unrepresentative data.

Example: The food-stamp program supports mostly freeloaders. Let me tell you about my worthless neighbor.

False analogy. Analogical arguments are tricky because there are almost always significant differences between the two things being compared. If the two things differ greatly, the analogy can mislead rather than clarify.

Example: You can't force a kid to become a musician any more than you can force a tulip to become a rose.

Either/or reasoning. This fallacy occurs when a complex, multi-sided issue is reduced to two positions without acknowledging the possibility of other alternatives. [I have heard this called "reductive fallacy," and I personally call it "false dilemma" —gpb]

Example: Either you are pro-choice on abortion or you are against the advancement of women in our culture.

Ad hominem ("against the person"). when people can't find fault with an argument, they sometimes attack the arguer, substituting irrelevant assertions about that person's character for an analysis of the argument itself.

Example Don't pay any attention to Fulke's views on sexual harassment in the workplace. I just learned that he subscribes to Playboy.

Appeals to false authority and bandwagon appeals. These fallacies offer as support for an argument the fact that a famous person or "many people" already support it. Unless the supporters are themselves authorities in the field, their support is irrelevant.

Example Buy Freebie oil because Joe Quarterback always uses it in his fleet of cars.

Example: How can abortion be wrong if millions of people support a woman's right to choose?

Non sequitur ("it does not follow"). This fallacy occurs when there is no evident connection between a claim and its reason. Sometimes a non sequitur can be repaired by filling in gaps in the reasoning; at other times, the reasoning is simply fallacious.

Example I don't deserve a B for this course because I am a straight-A student.

Circular reasoning. This fallacy occurs when you state your claim and then, usually after rewording it, you state it again as your reason. [This fallacy is also commonly called "Begging the Question" —gpb]

Example Marijuana is injurious to your health because it harms your body.

Red herring. This fallacy refers to the practice of raising an unrelated or irrelevant point deliberately to throw an audience off the track. Politicians often employ this fallacy when they field questions from the public or press.

Example: You raise a good question about my support for continuing air strikes in Bosnia. Let me tell you about my admiration for the bravery of our pilots.

Slippery slope. The slippery slope fallacy is based on the fear that one step in a direction we don't like inevitably leads to the next with no stopping place.

Example: We don't dare send weapons to these guerrillas. If we do, we will next send in military advisers, then a special forces battalion, and then large numbers of troops. Finally, we will be in all-out war.

Glossary of Causal Terms

[T]ake a closer look at some of the language that has grown up around causal debates.

The fallacy of the oversimplified cause. When conducting a causal analysis, a great temptation is to look for the cause of a phenomenon. But rarely is an event or phenomenon caused by a single factor; almost always, multiple factors work together. A carefully constructed causal analysis explains why one causal factor made a more or less important contribution than another; rarely does the writer try to convince the reader that a single factor is solely responsible for an effect.

Immediate versus remote causes. Every causal chain links backward to the indefinite past. Immediate causes are those closest in time to the effect you are studying; remote causes are those further away in time. An immediate cause of the LA riots would be the verdict in the Rodney King trial; a remote cause would be the launching of the Great Society programs in the sixties. The more remote in time the cause is from its purported effect, the greater the arguer's burden of proof. On the flip side, it is all too easy to place too much emphasis on an immediate cause and to treat it as the cause rather than as one among many.

Precipitating versus contributing causes. whereas remote and immediate causes have different temporal relations to effects, precipitating and contributing causes may coexist simultaneously. Contributing causes are the set of conditions that give rise to a precipitating cause, which helps trigger the effect. In the case of the LA riots, all the conditions that contributed to a sense of injustice in the affected community-lack of job opportunities, inferior schools, uneven police protection, and dwindling governmental support-could be considered contributing causes of the riots. All these conditions might have given rise to the anger that followed the verdict in the King case-the precipitating cause.

Necessary versus sufficient causes. A necessary cause is a cause that must be present for a given effect to occur. A sufficient cause is a cause that, if present, always triggers the given effect. For example, electricity in a circuit is a necessary cause for a refrigerator to run, but it is not a sufficient cause; other factors must also be present (a working motor, for example). A cause can be sufficient without being necessary Lack of water is a sufficient cause for a plant's death, but it is not a necessary cause (plants may die from other causes).

Constraints. Sometimes an effect occurs not because X happened, but because another factor, a constraint, was removed. A constraint is a kind of negative cause, a factor whose presence limits possibilities and choices. In the aftermath of the LA riots, many nearby communities immediately imposed curfews in their communities. Those curfews were constraints that prevented the riots from spreading.

[Remember the] post hoc, ergo propter hoc fallacy ("after this, therefore because of this"). The post hoc fallacy is the most common reasoning fallacy associated with causal arguments. It is tempting to assume that if event A occurs and event B follows, then event A must have caused event B. But chronological sequence does not guarantee causality.