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empties out by Sunday afternoon after brunch
is open again and you get your usual coffee. You
only a small crowd now shows up on Sunday nights.
feel fine the rest of the day. It must ve been Moe s
It must be that people don t like the new DJ.
coffee that gave me that stomachache yesterday,
you conclude.
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INDUCTIVE REASONING
2. You have a small dog, and you decide to take her Jason has been having trouble sleeping a few nights
to the new dog run in your neighborhood on a week. On the nights when he can t sleep, he notices
Monday morning. On Monday evening, your that the neighbor s dog is always barking and howl-
friend, who has just gotten a new puppy, asks if ing. Jason concludes that his trouble sleeping is due
she can bring the puppy to your house to play to the dog.
with your dog. On Tuesday morning, you notice
that you have several flea bites on your ankles. Jason has used a logical approach to determine
You conclude the cause of his insomnia. He s looking for a pattern
a. your dog picked up fleas at the dog run. something that is consistent with the nights he can t
b. your dog picked up fleas from your friend s sleep. Because he hears the dog barking and howling on
puppy. all of those nights, it could be that the dog is prevent-
c. either a or b. ing him from getting his sleep. The dog is the common
d. a and b. denominator for all of these occasions.
Just as it is important to be careful not to overlook
Answers other possible differences, however, it s important to
1. Yes, this is a logical casual argument. Whether it s remember to look for other possible common denom-
because there is a new DJ that doesn t have as big inators. Before Jason concludes that his sleeplessness is
a fan base as the previous one, or whether it s because of the dog barking, he should carefully con-
simply because the people don t like the music sider whether there might be anything else in com-
that the new DJ is playing, changing the DJ is mon on those nights that he can t sleep.
very likely to have caused the decrease in atten- So let s complicate the situation just a bit by
dance on Sunday nights. You should consider, adding more evidence from which to draw your
though, whether or not there have been other conclusion.
relevant changes in the caf, like new hours, new
management, or new prices. Jason has been having trouble sleeping a few nights
2. While all of these choices are possibilities, the best a week. On the nights when he can t sleep, he notices
choice is d. Your dog could just as easily have that the neighbor s dog is always barking. He also
picked up fleas from other dogs at the dog run as realizes that the sleepless nights are always nights
she could have from your friend s new puppy. Fur- that he hasn t talked to his girlfriend. Those are also
thermore, since your dog is exposed to both situ- nights that he skipped going to the gym because he
ations on the same day, she could have picked up worked late. What s causing Jason to have trouble
fleas both times. sleeping?
Looking for the Common a. the dog barking
Denominator b. not talking to his girlfriend
Sometimes, the cause can be determined not by look- c. not exercising
ing for what s different, but by looking for what s the d. none of the above
same that is, something that each incident has in
common. Take the following scenario, for example:
111
INDUCTIVE REASONING
Can you answer this question with confidence?
Post Hoc, Ergo Propter Hoc
Probably not. That s because each of these answers is a
legitimate possibility. Each situation occurs on the
Nina, who d always dressed rather plainly, decided it
nights Jason can t sleep. Just like the coffee wasn t the
was time to jazz up her wardrobe. She went shopping
only thing different in the previous scenario, here, the
and bought a closet full of new, brightly colored cloth-
dog isn t the only common denominator. There are
ing. Two weeks later, she was promoted at work. Wow,
many possibilities. If you re to confidently say which of
she told her friend, I had no idea that what I wore to
these is the cause, you need to pinpoint just one event
work could make such a difference. Just changing my
in common with all the bad nights.
wardrobe finally got me that promotion I d been wait-
If Jason knew that the dog barked every night
ing for!
even on those nights when he is able to sleep then the
Nina deserves congratulations, but not for her
barking dog could be eliminated as an option. Simi-
reasoning. What s wrong with her logic here?
larly, if Jason skips the gym on other occasions when
Nina has committed the post hoc, ergo propter
he can sleep, then choice c could be eliminated. But
hoc inductive reasoning fallacy. Post hoc, ergo propter
until more evidence is given and the other possibilities
hoc literally means after this, therefore because of this.
can be eliminated, none of the choices can be chosen
Nina has assumed that because her promotion came
over the others.
after she changed her wardrobe, her promotion was
caused by her change in wardrobe. Maybe, just
Practice
maybe, her appearance did have something to do
Read the following scenario and then answer the ques-
with it. But in all likelihood, there were several other
tions that follow.
causes for her promotion. She d probably been doing
good work for months or years, for one thing, and
It s summer and Barbara has been eating less than
the position to which she had been promoted may
usual. She notices that on the especially hot days, her
not have been vacant before. There may be several
appetite is low.
other reasons as well.
Of course, cause and effect is a chronological
3. Can Barbara say with confidence that the heat is
structure the cause must come before the effect
causing her to lose her appetite?
but remember that you need to consider other possible
causes. Just because A comes before B doesn t mean
4. What other possible common denominators
there s a logical connection between the two events.
could there be for Barbara s condition?
Here s another example of post hoc:
Answers
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