Unmasking Logical Fallacies

Defects that weaken arguments Look and sound like logic, but are not Learning to look for them strengthens your

ability to evaluate the arguments of others Learning to look for them strengthens the

arguments you make A word of caution, allowing your parents to

use logical fallacies in their arguments with you can sometimes work in your favor

Question: What does the graph below prove?

Answer: Nothing! The rising temperature of the earth is correlated to the declining number of pirates, but one did not cause the other.

CORRELATION DOES NOT MEAN CAUSATION!

CORRELATION: A complementary or parallel relationship between two data sets.

CAUSATION: Being responsible for an action or result.

When Natalie’s parents extended her curfew, her grades went up! You should extend mine, too!

http://www.youtube.com/watch?v=DQ24HdCPN9E&feature=related

"You can't trust Jones' theory of electromagnetic particles because he's a communist."

The arguer claims that a sort of chain reaction, usually ending in some dire consequence, will take place, but there's really not enough evidence for that assumption. The arguer asserts that if we take even one step onto the "slippery slope," we will end up sliding all the way to the bottom; he or she assumes we can't stop halfway down the hill.

Once we extend your curfew, you’ll ask us to do it again, then you’ll ask us to do it again and again!

If we grant a building permit to build a religious structure in our community, then there will be no bound on the number of building permits we will have to grant for religious structures and the nature of this city will change.

http://www.youtube.com/watch?v=K-wQI58szGw

Making assumptions about a whole group or range of cases based on a sample that is inadequate (usually because it is atypical or just too small). Stereotypes about people (“librarians are smart” or “wealthy people are snobs”) are a common example of the principle underlying hasty generalization.

ALL my friends get to stay out until midnight, just ask Jill – her parents let her stay out.

I don’t have an A, you don’t have an A, she doesn’t have an A – nobody gets an A in Mr. Fuller’s class.

“My roommate said her philosophy class was hard, and the one I’m in is hard, too. All philosophy classes must be hard!” Two people’s experiences are, in this case, not enough on which to base a conclusion.

One way of making our own arguments stronger is to anticipate and respond in advance to the arguments an opponent might make. In the straw man fallacy, the arguer sets up a wimpy version of the opponent’s argument and tries to score points by knocking it down. But just as being able to know down a scarecrow, or straw man, isn’t very impressive, defeating a watered down version of your opponent’s argument isn’t very impressive either.

This curfew argument is about trust. You don’t trust me to make my decisions. I don’t even know why you want me to go to college so bad if you don’t even trust me.

Senator Jones says that we should not fund the attack submarine program. I disagree entirely. I can't understand why he wants to leave us defenseless like that.

http://www.youtube.com/watch?v=DrnZdFFovBE

In false choice, the arguer sets up the situation so it only looks like there are two choices. The arguer then eliminates one of the choices so it seems we are left with only one option: the one the arguer wanted us to pick in the first place. But often there are really many different options, not just two – and if we thought about them all, we might not be so quick to pick the one the arguer recommends!

My boss wants me to work later. You can extend my curfew, or I’ll have to quit my job.

America: love it or leave it.

We’re playing against the Bulldogs tonight and they’re the best in the state, either we beat them and have some self respect, or we lose and hide our faces in shame forever.

An argument that begs the question and asks the reader to simply accept the conclusion without providing real evidence; the argument either relies on a premise that says the same thing as the conclusion (which you might hear referred to as “being circular” or “circular reasoning”), or simply ignores an important assumption that the argument rests on. Sometimes people use the phrase “begs the question as a sort of general criticism of arguments to mean that an arguer hasn’t given very good reason for a conclusion, but that’s not the meaning we are going to discuss here.

You’d be the coolest parents ever if you extended my curfew because that would be totally awesome.

Assisted suicide is morally acceptable because it is the right thing to do to help another human beings avoid suffering.

In appeal to ignorance, the arguer basically says, “Look, there’s no conclusive evidence on the issue at hand. Therefore, you should accept my conclusion on this issue.”

There’s no reason not to extend my curfew, and you can’t even think of a reason not to, so you should just extend it and get it over with.

I do not have much information on this except the general statement of the agency…that there is nothing in the files to disprove his Communist connections. – Joe McCarthy

The Latin name of this fallacy means “to the people.” There are several versions of the ad populum fallacy, but what they all have in common is that in them, the arguer takes advantage of the desire most people have to be liked and to fit in with others, and uses that desire to try to convince the audience to accept his or her argument. One of the most common versions is the bandwagon fallacy, in which the arguer tries to convince the audience to do or believe something because everyone else (supposedly) does.

80% of teenagers have a curfew of midnight or later; therefore, you should extend my curfew.

It is a fact that teachers’ pay must be tied to student achievement. Seven out of ten voters think so!

You should extend my curfew because my friends will desert me, my boyfriend will leave me, and my enemies will mock me if you don’t!

"I know the exam is graded based on performance, but you should give me an A. My cat has been sick, my car broke down, and I've had a cold, so it was really hard for me to study!"

"It's wrong to tax corporations—think of all the money they give to charity, and of the costs they already pay to run their businesses!"

You should extend my curfew because Academy Award Winner Sean Penn wrote in his recent book that curfews lead to rebellion later in life.

We should abolish the death penalty. Many respected people, such as actor Guy Handsome, have publicly stated their opposition to it.

http://www.youtube.com/watch?v=oyjla8Eoklg&feature=related

An argument in which the middle term is undistributed, meaning that not all the instances of things that are C are also instances of things that are A or of B. In other words, the first premise tells us that everything that is an A is also a C. It doesn’t tell us anything about whether things that are C are also things that are A.

Similarly, in the second premise, we are told that everything that is a B is also a C. But again, we know nothing about things that are C.A is a C. B is a C. Therefore A is a B. The argument is seductive because of its surface similarity to a valid argument form: A is a C. C is a B. Therefore A is a B. In this argument, we know something about A (namely, that every instance of A is also an instance of C). And we also know something about C (namely, every instance of C is also an instance of B). Since the C is distributed in the second premise, we can correctly link A with B.

EXAMPLE: Most Arabs are Muslims and all the 9/11 hijackers were also Muslims. Therefore most Arabs are hijackers.

ANALYSIS: The conclusion doesn’t follow from the premises. To show this, substitute the following argument: My 5-year-old enjoys watching television, and teenagers also enjoy watching television. Therefore my 5-year-old is a teenager.

EXAMPLE 1: Baptists are often politically conservative.

(premise) Republicans are also often politically conservative.

(premise) Therefore Baptists are often Republicans.

(conclusion) EXAMPLE 2:

John Elway is a great quarterback. (premise) Michael Vick is a great quarterback. (premise Therefore, Michael Vick is really John Elway.

(conclusion)

http://www.youtube.com/watch?v=zrzMhU_4m-g

“She’s a Witch!” from Monty Python and the Holy Grail.