In this sections, we'll consider a number of fallacies, i.e., common errors in reasoning which people (even well educated, careful people) often fail to notice in their own arguments or which devious people might use in their arguments in the hope that we won't notice them.

The practice of identifying and classifying fallacies has been a popular one since Aristotle's time.  Here, we'll consider a relatively small group of commonly discussed fallacies, most of which are likely to rear their heads again later on in this course.  If you're curious about other fallacies, there are some pages about them on the web.  See, for instance, Charles Ess' page on fallacies.

What follows then is a collection of bad arguments.  In most cases, you'll quickly see they're bad arguments, but what you should think about is exactly what it is that makes them bad arguments.  Each of these arguments can be seen as an example of a particular type of fallacy.  When you've thought for a bit about why the argument is a bad one, click on the text that says "What's wrong with this argument?" and you'll get a description of the fallacy involved.

Something to keep in mind:  There may be more than one way of saying what's wrong with each argument, so you shouldn't necessarily worry if you come up with a different answer than I did (although it would be a good idea to run your answer by me).  You should, however, make sure you understand the explanation that's given of what's wrong with the argument (and so of the type of fallacy the argument involves).

What's Wrong With These Arguments?

1. If you came to every class then you passed the course.  But you did not come to every class, therefore you did not pass the course.

[What's Wrong With This Argument?]

2.  If you came to every class then you passed the course.  As a matter of fact, you did pass the course, therefore you must have come to every class.

[What's Wrong With This Argument?]

3. As scientists have shown us, the world is a well regulated place. They have discovered many laws of physics (such as the law of mass conservation). But everyone knows that you cannot have laws without a lawmaker. There must therefore be a Supreme Lawmaker. In spite of their anti-religious tendencies, then, scientists have actually helped to prove the existence of God.

[What's wrong with this argument?]

4. Students must decide whether they will have fun or do well. This student has decided to have fun, so I guess he won't be doing well.

[What's wrong with this argument?]

5.  Everything in the Bible must be true.  The Bible says it is the word of God and God would never lie.

[What's wrong with this argument?]

6. If this doctor isn't punished for helping his patient to commit suicide, we send doctors the message that they can engage in euthanasia without fear of prosecution.  If that happens, cases of euthanasia will become common.  Thus, the public and the medical profession will become used to the idea of taking human life.  Physicians will lose their natural reluctance to put patients out of their misery. Soon, assisted suicide will become commonplace, just another lifestyle choice.

[What's Wrong With This Argument?]

7. Successful people usually have large vocabularies.  It seems then that having a large vocabulary must make one likely to be successful.

[What's Wrong With This Argument?]

8. Some people argue that capital punishment is necessary, because it is the only appropriate punishment for those who commit the most atrocious and depraved crimes. However, we should not pay attention to such arguments. It is well established that those who argue for capital punishment are usually people who see themselves as helpless and inadequate in a changing complex world. Subconsciously, such people see capital punishment as a way of striking back at a confusing world.

[What's Wrong With This Argument?]

9. People who think abortion should be banned have no respect for the rights of women. They treat them as nothing but baby-making machines. That's wrong. Women must have the right to choose.

[What's Wrong With This Argument?]

1. If you came to every class then you passed the course.  But you did not come to every class, therefore you did not pass the course.

P1: If you came to every class, you passed the course.
P2: It is not the case that you came to every class.
C: It is not the case that you passed the course.

This argument commits the fallacy of denying the antecedent.  Notice that premise 1 does not say that the only way to pass the course is to come to every class.  The argument does nothing to rule out the possibility that a person who, despite skipping some classes, gets 90% on every test, assignment and exam will pass the course.

In general, any argument that has the form:

P1: If P then Q.
P2: It is not the case that P.
C: It is not the case that Q.

commits the fallacy of denying the antecedent.

 
[Back to the list of arguments]


2.  If you came to every class then you passed the course.  As a matter of fact, you did pass the course, therefore you must have come to every class.
 

P1: If you came to every class, you passed the course.
P2: You passed the course.
C: You came to every class.


This argument commits the fallacy of affirming the consequent.  Even if it is true that I will pass anyone who came to every class and it is also true that you passed the course, you may be that smart person who, despite skipping some classes, got 90% on every test, assignment and exam (and so passed).

In general, any argument that has the form:

 
P1: If P then Q
P2: Q
C: P

commits the fallacy of affirming the consequent.

For more on the fallacies of affirming the consequent & denying the antecedent see 'The AB47 Example'.


 
[Back to the list of arguments]


3. As scientists have shown us, the world is a well regulated place. They have discovered many laws of physics (such as the law of mass conservation). But everyone knows that you cannot have laws without a lawmaker. There must therefore be a Supreme Lawmaker. In spite of their anti-religious tendencies, then, scientists have actually helped to prove the existence of God.

This argument commits the fallacy of equivocation. In the second sentence, law means something like 'natural regularity'. In the third sentence, it means something like 'a command made by a governing body or person'. As long as we give the word 'law' the right definition in each premise, the premises are certainly acceptable. However, the arguer acts as though 'law' means the same thing in both sentences. Now, if it did, the argument would be valid, so there's a good strategic reason why the arguer would pretend that law means the same thing in each premise. The problem, however, is that if we try to use the same definition of 'law' in both premises, we no longer have a good reason to think both premises are true. In order to fool us, what the arguer needs to do is have us think 'law' means a different thing in each sentence when we're thinking about whether the premises are true, but not when we're thinking about whether the argument is valid.

In general, a person commits the fallacy of equivocation, when a word or phrase in her argument has more than one distinct meaning and her argument relies on this, but she is disguising the fact that this is so (or at least fails to realize this).

[Back to the list of arguments]


4. Students must decide whether they will have fun or do well. This student has decided to have fun, so I guess he won't be doing well.
 

The structure of the argument seems to be:
 
P1: Students can either do well or have fun (but not both).
P2: This student is having fun.
C:  This student won't do well.

But surely P1 misrepresents the alternatives.  Why can't a person do both?  When you misrepresent the alternatives in this way, you commit the fallacy of false dichotomy.  In general, the fallacy of false dichotomy occurs when an argument presents a set of alternatives as though they are exclusive or exhaustive (or both) when in fact they are not.

Exclusive Alternatives:  alternatives are exclusive when they are such that if one holds the other cannot, e.g., being square and being circular are exclusive alternatives.  Notice that argument #2 presents doing well and having fun as exclusive alternatives although they are not.

Exhaustive Alternatives:  alternatives are exhaustive when they cover all the possibilities, e.g., being pregnant or not being pregnant.
 


[Back to the list of arguments]

5. Everything in the Bible must be true.  The Bible says it is the word of God and God would never lie.
 

This argument commits the fallacy of begging the question.  The argument seems to be:
 
P1:  The Bible says it is the word of God.
P2: God would never lie.
C: Everything in the Bible must be true.
 
The problem is that this argument would only be convincing if we assumed that the Biblical claim reported in P1 is true.  But this means that the argument is only convincing if we assume that what the Bible tells us is true.  That's the very thing we were supposed to be proving in the first place!  Thus, this argument engages in circular reasoning.

In general, an argument begs the question when it implicitly or explicitly assumes the truth of whatever it is supposed to be proving, i.e., when it engages in circular reasoning.
 

[Back to the list of arguments]

6. If this doctor isn't punished for helping his patient to commit suicide, we send doctors the message that they can engage in euthanasia without fear of prosecution.  If that happens, cases of euthanasia will become more common.  As a result, the public and the medical profession will become used to the idea of taking human life.  Physicians will lose their natural reluctance to put patients out of their misery. Soon, assisted suicide will become commonplace, just another lifestyle choice.
 
This passage commits the very common slippery slope fallacy.  It makes a dubious case that some relatively minor event will set in motion a chain of events that will eventually lead to a catastrophic result (and so makes a case that the relatively minor event should be prevented).  Of course, not every argument that has this form is fallacious.  Sometimes minor events do have major consequences.  In fact, notice that an argument with the classic slippery slope form:
P1: If X happens then Y will happen.
P2: If Y happens then Z will happen.
P3: If Z happens then A will happen.
C: If X happens then A will happen.

is deductively valid. So, when does an argument of this sort commit the slippery slope fallacy? When one of the following two problems exists:

1. Weak Links: The chain of reasons may have a weak link. You need to be very careful when stringing together a series of claims with the form 'If X happens, then Y will happen'.  If one of those claims turns out to be false, then the argument fails to be convincing. [One way of thinking about why this is so is to note that in a slippery slope argument none of the premises work independently. They stand or fall together.]

2. Magnifying Uncertainty: If your argument has the form:
 

P1: If X happens then Y will happen.
P2: If Y happens then Z will happen.
P3: If Z happens then A will happen.
C: If X happens then A will happen.
 
then any uncertainty about the relationship between X and Y combines with the uncertainty about the relationship between Y and Z and so on.  In other words, the uncertainty involved in each step gets compounded, so that, even if each premise is quite plausible by itself, it may be very uncertain whether the first event will really lead to the last one.  (You might think of an example here.  If you flip a coin once your chance of correctly guessing whether heads or tails will come up is 50%.  That's a pretty good chance.  However, as we add in more 'steps' your chances falls quickly.  Your chance of guessing correctly twice in a row is only 25%.  Your chance of guessing three times in a row is 12.5% and so on.  Thus, the uncertainty in each step combines into a great deal of uncertainty.)

If an argument runs into trouble on either of these counts, it commits the slippery slope fallacy.
 

[Back to the list of arguments]

7. Successful people usually have large vocabularies.  It seems then that having a large vocabulary must make one likely to be successful.
 

This argument commits a causal fallacy. A causal fallacy occurs when we mistake an association or correlation between two things or events for a causal relationship between those things. In the example, what seems to be going on is that the person making the argument has confused something that might be associated with success (i.e., having a large vocabulary) with something that causes success.  A better explanation of this association might be that both success and having a large vocabulary are caused by the same thing, e.g., education.

Here is another example of a casual fallacy:
 

In one of Aesop's fables, a rooster notices that every morning right after it crows, the sun rises. The rooster concludes it must be the greatest creature in the world since it causes the sun to rise. 

Here, while it is true that there is a correlation between the rooster crowing and the sun rising it is clearly a mistake to think that the rooster crowing causes the sun to rise. This particular sort of fallacy is so common that it has its own name. The rooster has committed the post hoc fallacy, i.e., the fallacy of assuming that since event A always precedes event B, event A must cause event B.

[Back to the list of arguments]

8. Some people argue that capital punishment is necessary, because it is the only appropriate punishment for those who commit the most atrocious and depraved crimes. However, we should not pay attention to such arguments. It is well established that those who argue for capital punishment are usually people who see themselves as helpless and inadequate in a changing complex world. Subconsciously, such people see capital punishment as a way of striking back at a confusing world.

This commits the fallacy of ad hominem. It attacks the arguer not the argument.
 
[Back to the list of arguments]

9. People who think abortion should be banned have no respect for the rights of women. They treat them as nothing but baby-making machines. That's wrong. Women must have the right to choose.
 

This argument commits the straw man fallacy. It misrepresents the position of the opposing side in such a way that the opposing position appears obviously false or ridiculous. (That is, it creates a 'straw man' just so it can be knocked down.)  It's worth noting that 'straw manning' often takes place on both sides of charged debates like the one concerning abortion.  Consider a claim like: 'People who are in favour of abortion think it's okay to commit cold blooded murder.'
 
[Back to the list of arguments]

[Philosophy 1200]