Practice Questions & Solutions - Fallacies
Philosophy 1200-002
Andrew Latus

What fallacies are committed in the following passages?  Explain.

1. How can anyone seriously believe in evolution? I certainly donít. How can you take seriously a theory that claims humans are just monkeys with less hair and that our ancestors were apes?

This commits the Straw Man fallacy.  Evolutionary theory just doesn't claim that humans are monkeys with less hair.  That's a vast oversimplification of the theory, hence you've created a 'straw' version of the original theory.
2. A recent study showed that students who cram immediately before examinations usually get lower grades than those who do not. Well, I certainly wonít make that mistake this term. Iím not even going to open a book during the exam period.
The student seems to be under the impression that because cramming tends to come before having a bad grade, it's the cause of the bad grade.  But that's not correct, what causes the bad grades is putting yourself in a position such that you need to cram.  Once you've done that you might as well cram as it will minimize the damage that not working all term does.  The student has assumed that because a particular thing (i.e., cramming) tends to come before bad grades, it's the cause of the bad grades.  Thus, the student is commiting a causal fallacy (in particular, the post hoc  fallacy).
3. If you don't get to bed early, you'll be too tired to do well on the GRE tomorrow.  If that happens, you won't get accepted into a decent graduate school. Youíll end up a washed-out alcoholic living in a trash-bin.
This is a pretty clear instance of the slippery slope fallacy.  A whole load of 'if-then' claims are made here (and some others seem to be assumed), that together make a very weak case for the ultimate conclusion of the argument.
4.Two weeks after they put fluoride in St. Johnís water I got sick. I had a fever and couldnít keep solid food down. So, I switched to drinking bottled water and I recovered in a day or so. That fluoridated water is dangerous stuff.
This is another instance of a causal fallacy (again, a post-hoc fallacy).  Just because the flouridation of the water preceded the headache and the headache cleared up after he stopped drinking flouridated water, we can't assume that the flouride caused the headache.  We'd need much better evidence than that.


5.War is the only way of stopping Saddam Hussein. After all, sanctions havenít worked at all.

This seems to assume a false dichotomy, i.e., it seems to assume that the only options are war or sanctions.  But what about diplomacy or other possibilities?  In other words, the argument assumes that the possibilities of sanctions and war exhaust all the possibilities, but they don't.  As such it commits the fallacy of false dichotomy.


6. If the police knew that Bobo had a motive for the crime then he would be a suspect.  But the police do not know that Bobo had a motive, therefore he's not a suspect.

Notice that the first sentence doesn't say 'Bobo would only be a suspect if the police knew he had a motive for the crime.'  As such, we can't conclude that because they don't know this, he's not a suspect.  There may be other reasons to consider him one.

The argument has the form:

P1: If A then B
P2: It is not the case that A
C: It is not the case that B

As such, it commits the fallacy of denying the antecedent.

7. Undemocratic societies kill the human spirit. The reason is clear: unless the people have the power in their society,the human spirit withers.

This is a pretty standard instance of begging the question. The conclusion seems to be that undemocratic societies kill the human spirit, but the only reason we are given to believe this is essentially that same conclusion in different terms.
8. Darwin's theory of evolution is just that, a theory.  Theories are just speculation with no evidence behind them.  We don't want our children to learn theories with no evidence behind them, so we shouldn't allow the theory of evolution to be taught in school.
This commits the fallacy of equivocation.  We do sometimes use the word 'theory' to mean a speculation with little or no evidence behind it, but that's not the way we're using the word when we describe the theory of evolution as a theory.  Hence, a word is being used in 2 quite distinct ways here, but the arguer is acting as though he is using the word consistently throughout the argument.
9. Anyone who teaches philosophy is an obnoxious freak, so that obnoxious freak over there must be a philosophy teacher.
This affirms the consequent.  Even if it is true that all philosophy teachers are obnoxious freaks, it doesn't follow that all obnoxious freaks are philosophy teachers.

Another way of expressing the argument is:

P1:  If you teach philosophy then you are an obnoxious freak.
P2:  The person is an obnoxious freak.
C: That person must teach philosophy.

But that argument is invalid for the reason mentioned above.


10.  What Peter Singer said about us needing to give more to charity can't be right.  After all, Singer is just another one of those obnoxious freaks.

This is a case of a clear ad hominen fallacy.  Singer's argument isn't attacked here, Singer is.
[Philosophy 1200]