One aspect

of the art 

of critical 

thinking is 

the ability to 





Logical Fallacies
Understanding valid arguments
 By Dave Clarke 

The author is an O4R member and serves as our video librarian. He attends Eastern Oregon University.

Lord Byron once said that he who cannot reason is a fool and he who dares not is a slave. Let’s face it. The ability to think critically is vital. Without critical thinking, you and your money are targets for every con-artist, quack, and late-nut radio host hawking laundry balls, crystals and magnetic shoe inserts. You may spend a fortune on worthless advice from psychic readers or UFO-alien books written by authors seeking the fast buck instead of the facts. Lack of critical thinking skills could even cost you your life. Remember the Heaven’s Gate cult?

     Critical thinking is an art. One aspect of the art of critical thinking is the ability to recognize logical fallacies. A fallacy is a mistaken belief or an unsound argument that follows from errors in reasoning or inference. The list of fallacies is long, so I will discuss only some of the most common. Understanding these fallacies will equip you to distinguish strong, logical arguments from specious, weak or invalid ones. And, will provide you with tools to identify those elusive things called “true statements,” which really are “gems found at great depths.”

The hasty generalization

     A hasty generalization occurs when we draw a conclusion based upon little evidence. Suppose, for example, I meet a group of youths who are both gang members and not of my own race, and then hastily conclude that all young people of this particular race are affiliated with gangs. Of course this would be wrong. Hasty generalizations often form the basis of racial prejudice. It is unjustified to jump to such a conclusion from a small number of experiences.

     Hasty generalizations also arise from misunderstanding cause and effect. These misunderstandings form the basis of many superstitions. We’ve all heard the old superstition concerning cats, who have at various times and places been regarded as either a holy or an evil omen. Their bad rap came from the belief that cats were the spirits of witches. Bad luck supposedly comes to anyone who has a black cat cross his path from left to right. (In East Yorkshire, many believe that owning a black cat is lucky, but meeting one isn’t.) My black cat, Samantha, regularly runs across my path from left to right and right to left. I’ve even met a few stray black cats, but bad luck? Nay! Do not make unjustified connections. It is unwise and unjustified to assume a cause and effect relationship without an identifiable causal mechanism.

False analogy

     By definition, an analogy is a partial similarity. Philosopher S. Morrison Engel stated “when we argue by analogy, we attempt to explain facts that are obscure or difficult by comparing them to facts that are already known or better understood and to which they bear some likeness.” Consider the skin lotion advertisement that tells you, “You’ve seen land crack when it becomes dry. The same thing can happen to your skin when it loses its moisture.” Notice how the advertisement seizes upon the similarities of dry, cracked land, which most of us have seen, and dry, cracked skin. You are supposed to conclude that what is true of one will also be true of the other. But in this case, the resemblance is trifling and the differences are significant.

     Not all analogies are false analogies. An analogy can be useful for illustrative purposes. It can give color to a position supported in other ways. But see it for what it is, and don’t give it more weight than it deserves.

Appeal to authority

     A fallacy based upon an appeal to authority goes something like this: Jones states that X is true because Smith, who is an authority, says it’s true. The problem is that while Smith may have a great deal of prestige, Jones has based his argument on Smith’s authority rather than on Smith’s evidence. Facts, not authorities, confirm or disconfirm statements.

     I might, for example, argue that Uri Geller has supernatural powers because learned physicists have seen him perform and believe he has demonstrated great feats of mind over matter. We must recognize, however, that physicists are also capable of being deceived. On June 21, 1974, at Birkbeck College in London, Geller performed in front of several physicists, including quantum mechanics expert David Bohm. Geller held a Geiger counter tube and caused a strong burst of activity as he showed signs of strained breathing and great physical exertion. He repeated the demonstration the next day for writers Arthur Koesteler and Arthur C. Clarke. Both were astonished. Jack Sarfatti, a Ph.D. physicist stated, “Geller demonstrated genuine psychoenergetic ability at Birkbeck, which is beyond the doubt of any reasonable man.” (It should be noted, however, that Clarke, Sarfatti and others later changed their minds about Geller.)

     The real authorities in this case were conjurers well-versed in the art of deception. The many physicists and engineers who became convinced of Geller’s powers did not have the insight of magician James Randi, who exposed Geller in his book The Magic of Uri Geller. Arguments must be based on proper insight and evidence, not on the education, intellect, or prestige of an individual.

Argument from ignorance (argumentum ad ignorantiam)

     I like to call the argument from ignorance “the mimic.” That’s because the argument “mimics” a legitimate type of argument. I might say that, under specific conditions, pure water melts at 32°F. This is a legitimate statement because a) there is evidence that shows this to be true and b) there is no evidence showing it to be false. Not once has ice not melted at this temperature under these conditions. Arguments from ignorance mimic this form by declaring a statement to be true because there is no evidence against it. Statements may indeed have no evidence against them, but this is not enough to prove they are true. A statement must also have evidence for it. I could say that little green elves live in my garage. There certainly is no evidence against the idea. There might be a fairy there too. Or a Bigfoot, or lots of Bigfoots (Bigfeet?). There is, however, a lack of evidence to support these ideas and, therefore, my statement regarding the elves is simply an argument from ignorance. It is inconclusive.

Begging the question (petitio principii )

     If you assume the very premise you’re trying to prove, you are “begging the question.” This fallacy is also known as “arguing in a circle,” or “circular reasoning.” The purpose of deductive reasoning is to get from one point in an argument (the premise) to another (the conclusion) in a logical manner. But a circular argument does not allow this progression. If the premise or beginning of the argument is identical to its conclusion, the argument doubles back on itself and becomes barren.

     In his excellent book Introduction to Logic, Irving Copi offers the following example of a circular argument that clearly begs the question:

   “ may argue that Shakespeare is a greater writer than Robbins because people with good taste in literature prefer Shakespeare. And if asked how one tells who has good taste in literature, one might reply that such persons are to be identified by their preferring Shakespeare to Robbins.”
     In other words, A equals A. The person making the argument for Shakespeare should provide evidence for his assertion, instead of merely repeating it.

Non sequitur

     The Latin term non sequitur means “does not follow.” The fallacy of non sequitur is also known as an “argumentative leap.” Immanuel Velikovsky’s book Worlds in Collision is a widely challenged work on evolutionary development and the origin of the Earth. One advertisement for the book states, “Once rejected as ‘preposterous!’ Critics called it an outrage! It aroused incredible antagonism in scientific and literary circles. Yet half a million copies were sold, and for twenty-seven years it remained an outstanding bestseller.”

     Yes, the book was widely popular, but was it scientifically respectable? No. Many copies may have been sold, but that is irrelevant to the book’s theoretical soundness.

Slippery slope (reductio ad absurdum )

     The slippery slope fallacy is also known as the “unjustified projection.” The argument is made that a first step will inevitably lead to another, usually undesirable, final outcome. If there is inadequate evidence that this outcome will follow, beware-you are facing a slippery slope fallacy. Taking an argument all the way to the end is often a great way to determine the argument’s validity. Demand evidence that one outcome necessarily follows from the first step.

     We don’t know that a ban on handguns will lead to a ban on hunting rifles and other weapons. We can’t say that legalizing abortion will lead to murdering the old and the physically and mentally handicapped. What evidence supports these outcomes? Each argument rushes downhill to a conclusion that is not supported by valid evidence. The slippery slope argument is tricky because it relies on future events, and only future events will verify or refute the prediction. Arguments that project unjustifiably are also simplistic because they ignore the dissimilarities between first and last steps, and because they ignore the complexity of developments in any long chain of events.

Straw man

     A claim is often a series of arguments. If I take the weakest link in a series of arguments and refute only that link, and then claim to have refuted the entire argument, I’d be guilty of the straw man fallacy. The name “straw man” probably came from an old game where a straw man was set up to divert attention away from the real opponent. A straw man argument is created by redefining an opponent’s position, molding it into an easily refutable target, and then arguing against this misrepresentation of the position. It’s a diversionary tactic-a means of stacking the deck in your favor by creating a target you’d like to argue against.

     Former Senator (later President) Richard Nixon left us with an excellent example of a straw man argument in his famous “Checkers” speech given during the 1952 presidential campaign when he was a vice-presidential candidate. Accused of misappropriating $18,000 in campaign funds for his personal use, Nixon defended his reputation with the following speech.

   “One other thing I probably should tell you, because if I don’t they will probably be saying this about me, too. We did get something, a gift, after the election. A man down in Texas heard Pat on the radio mention the fact that our two youngsters would like to have a dog and, believe it or not, the day before we left on this campaign trip we got a message from Union Station in Baltimore saying they had a package for us. We went down to get it. You know what it was? It was a little cocker spaniel dog in a crate that he had sent all the way from Texas, black and white spotted, and our little girl, Tricia, the six-year old, named it Checkers. And, you know, the kids, like all kids, loved the dog, and I just want to say this, right now, that regardless of what they say about it, we are going to keep it.”
     So what does the dog have to do with the alleged misappropriation of $18,000 dollars of campaign funds? Nixon built a straw man. It was the money-not the dog-that was at issue!

Ad hominem

     This very common logical fallacy redirects the focus of the argument, moving it from the argument itself to the person making the argument. In Latin, ad hominem means “against the man.” Basically, it’s a personal attack. According to Skeptic magazine publisher Dr. Michael Shermer, the goal of the ad hominem attack is to “discredit the claimant in hopes that it will discredit the claim.” If one can attack the author of a statement instead of attacking the statement, attention might be diverted from the real issues at hand. Ad hominem attacks, however illogical, whip up an attitude of disapproval. We see a lot of aspersions cast in the area of politics-commonly called “mudslinging.” A Democratic senator might say, “You can’t trust Republican Senator X. Two years ago he was an advocate of the position he now opposes.”

     Sounds good, right? The Democratic senator seeks to shift the focus away from the argument to Senator X whom we “can’t trust” due to his erratic behavior. This shift is a sidestep. It might win votes, but it’s not logical. The Democratic senator should have gathered facts and shown why those facts dispute his opponent’s position. The fact that his opponent changed his mind on an issue is irrelevant. A position change doesn’t transform a person into someone who cannot be trusted. And even if Senator X is an erratic scoundrel, it doesn’t matter; a scoundrel may argue validly and speak the truth.


Copi, Irving M. 1978. Introduction to Logic. New York: Macmillan.

Dauer, Francis W. 1989. Critical Thinking. New York: Oxford University Press.

Engel, S. Morris. 1981. The Study of Philosophy. New York: Holt, Rinehart, and Winston.

Radford, E. 1961. The Encyclopedia of Superstitions. New York: Helicon.

Randi, James. 1975. The Magic of Uri Geller. New York: Ballantime.

Rottenberg, Annette T. 1996. Elements of Argument. Boston: Bedford Books.

Shermer, Michael. 1997. Why People Believe Weird Things. New York: W.H. Freeman and Company.

Weston, Anthony. 1987. A Rulebook for Arguments. Indianapolis: Hackett.

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