Spotting Common Logical Errors Can Make You a Better Critical Thinker

When I was in my late teens and early twenties, I would occasionally get into a debate about some religious or political topic, and I would often end up very frustrated. At times, I would just know that I was right but would have trouble explaining why. Or, I would read an opinion piece in a newspaper or magazine, and I would know there was something wrong with the author's argument, but I couldn't put a finger on exactly what was in error.

Later, when I took a basic logic course, I realized why many of my debates had been so frustrating and fruitless. Learning the formal logical fallacies made me a much better debater and a much savvier consumer of news and opinion. Knowing how to spot a few common logical errors can work for you, too, and you won't even have to pay tuition to take a philosophy class at the university.

Becoming familiar with these fallacies can help you hone your "B.S. detector" skills. Learning these logical errors can also help you assess the claims of people marketing a product more clearly, help you keep calm during a debate with a friend or relative, and will make you a better all around critical thinker. Below are seven examples of common fallacies that pop up from time to time in real life.

Number One: The Personal ("ad hominem") Attack.

When you attack the source of the claim or scientific study rather than addressing the facts, you're committing a logical fallacy. Recently, Texas legislators decided to mandate HPV vaccines for girls in grade school. Some criticized the decision saying that these lawmakers were influenced by money by big pharmaceutical companies. However, those people were committing an ad hominem (Latin for "against the man") or personal attack. They were attacking the credibility of the source, rather than addressing whether mandating the HPV vaccines was a good idea based on the science and the facts.

This applies to any scientific study you are wary of just because it's funded by a group or corporation with a vested interested. It's important to know your sources of information and any possible conflicts of interest-- that's true. It's good to be skeptical about your sources. However, just because you don't like the source or are concerned about possible biases doesn't mean the study or proposal is wrong or bad. To avoid committing a logical fallacy, you need to argue on the merits of the proposal or the legitimacy of the science, rather than on the reputation of the messenger.

Number Two: Confusing Correlation with Causation.

The classic example of this fallacy is the assertion "When ice cream sales are high, so are murder rates. So ice cream consumption must cause murders." Of course, there's no causation there; ice cream sales and murder rates just both happen to go up when the weather is hot.

This fallacy comes into play in anti-vaccination arguments because of the supposed correlation between exposure to mercury in vaccines and the rising rates of autism. As more children have been vaccinated, the argument goes, more cases of autism have occurred. Therefore, the mercury in vaccines must be causing autism. There are no well-constructed studies that show any link between the mercury in vaccines and autism. What is more likely is that over the years, screening has been catching more cases of autism in children and the criteria for diagnosing mild forms of autism have broadened, resulting in larger numbers of children being diagnosed as autistic, rather than many, many more causes of autism occurring than in previous years.

On the other hand, you can't entirely ignore strong correlations in medical studies; this is exactly how we found out that smoking tobacco causes cancer.

Number Three: The Appeal to Emotion or Pity.

When pro-life advocates put up big photos of mangled fetuses, they're appealing to your feelings of pity, rather than arguing about abortion law based on facts.This fallacy is really easy to commit, in part because it's so pervasive that we often don't notice it. Including a bit of emotional appeal is a long-standing tactic in debates, political speeches, and in persuasive speaking generally. For instance, what politician hasn't made an emotional appeal about how some issue will affect our children and grandchildren? Emotional appeal can be an effective technique, but that doesn't make it a logical argument.

Number Four. Relying on Anecdotal Evidence.

If someone is making a universal statement, such as "All Volvos are always reliable", finding and pointing out a single counter-example is a valid way to reply to the argument. But, often, anecdotal counter-examples are raised as if they somehow invalidate a generalization, such as "Volvos, on the whole, tend to be reliable, and they're rated highly by Consumer Reports." Merely hearing one counter-example about a lemon Volvo does not invalidate the generalized statement that Volvos, in general, tend to be more reliable than most other makes of car.

Say for instance I make the statement that, statistically speaking, overweight and obese people who exercise regularly have about the same mortality rates and risk for many diseases as thinner active people, and they have much better health than thin, sedentary people. Someone may come along and cite the case of her Aunt Mary who was 350 pounds and could barely get out of bed and died at age 52 of heart disease. This is as fallacious an argument as someone else who says, "Oh yeah? Well my Grandpa Mortimer weighed 375 pounds, and he was healthy as a horse up until about the last month or two of his life, and he lived to be 90!" Anecdotal evidence cannot be used as a valid way to attack my argument. You must provide other scientific studies, preferably with a large sample size, that would show the opposite of the claim I've made.

Number Five. The Argument from Ignorance.

The "argument from ignorance" goes something like this:

"Statement X can't be true because there's no proof for it."
or
"Statement Z must be true because there's no proof that it isn't true."

An example might be, "Psychic phenomena don't exist because there's no proof that they're real." There might be other, valid, arguments against the existence of psychic phenomena, but the one presented above isn't a valid argument.

Another example would be, "God exists because nobody has proven that he doesn't." Of course, it's as consistent (and equally fallacious) to argue, "Oh yeah? Well, God doesn't exist because nobody has proved that he does."

Number Six: The Spotlight Fallacy.

This fallacy is just a variation on the "hasty generalization." If you think that all homosexual men are flamboyant drag queens and all lesbians wear leather and ride motorcycles because you've seen one gay pride parade on TV, you're falling victim to the "spotlight" fallacy. This fallacy shows up in the media frequently. A recent example would be stereotyping all Muslims as violent because of a few bad apples who are in the media spotlight.

Number Seven: The Appeal to Antiquity or Tradition.

The appeal to antiquity is often used to support the legitimacy of mystical cures or alternative medical treatment, such as faith healing or acupuncture. However, a few minutes of reflection will make it apparent that just because some belief is old or longstanding doesn't make it true. After all, it was believed for millennia that the earth was the center of the universe, but that doesn't mean the theory of "geocentrism" is correct.

A more extensive list of fallacies and examples of each can be found at the Logical Fallacies Info page.