(intro music)
Hello, I'm Paul Henne,
and I'm a philosophy graduate student
at Duke University.
And in this video I'm gonna talk to you
about an informal fallacy
called the "fallacy of division."
And I'm also going to[br]discuss why it's wrong
to conclude that water molecules are wet.
The fallacy of division[br]is a defect in reasoning
that arises when someone[br]infers that what is true
of something must also be[br]true of that thing's parts.
So, the fallacy is kind of like saying
that because a university[br]has some qualities,
then all of that university's departments
must also have those qualities.
And that's a fallacy[br]because even if a university
is good all around, it doesn't necessarily
mean that the university[br]has a good taxidermy
program, for instance.
So, sounds problematic, right?
But let's represent the[br]logical error more formally.
The reasoning is something like this.
Premise (1): Whole A has[br]properties A, B, and C.
Premise (2): P is a part of A.
Conclusion: Therefore,[br]P must have properties
A, B, and C.
The argument seems attractive,
but the style of argument will not always
lead to true conclusions,
for it might not be the case
that the parts and the whole[br]have the same qualities.
You may now be able to see
what's going wrong with[br]this line of reasoning.
So, let's look at a few examples.
Suppose that I have a car,
and I made this argument about my car.
Premise (1): My car is red[br]and it goes really fast.
Premise (2): The muffler[br]is a part of my car.
Conclusion: Therefore, my car's muffler
is red and goes really fast.
Okay, that was an easy one.
Of course no one would make this argument,
or assume that I own a car,
but it demonstrates where[br]the style of argument fails.
Clearly the muffler doesn't have
the same properties as the car.
So, let's try another[br]more familiar example.
Suppose that your friend[br]made this argument:
the computer is indestructible;
the hard drive is a part of the computer;
therefore, the hard[br]drive is indestructible.
Now, suppose your friend's[br]premises are true.
The conclusion could still be false.
Maybe your friend was making this argument
in order to transfer the hard drive
of the indestructible computer[br]to a different computer.
So, if would be unfortunate[br]for her to find out
that the hard drive is only indestructible
when it's connected to[br]the other components
of the original computer.
So, the hard drive itself[br]isn't indestructible
like the whole computer.
Your friend committed[br]the fallacy of division.
That is, to be precise,[br]she hastily assumed
that the hard drive, the[br]part, is indestructible
like the whole computer.
So, we just learned about[br]the fallacy of division,
or the error in reasoning that comes about
when a person infers that[br]what is true of something
must also be true of its parts.
It is important, however,[br]to note that this
fallacious reasoning doesn't always lead
to a false conclusion.
I, for instance, might[br]argue the following.
Premise (1): The house in pink.
Premise (2): The front[br]door is part of the house.
Conclusion: Therefore, the[br]front door is also pink.
And in this case, my argument leads
to a true conclusion.
For I've, for whatever reason,
painted every part of my house pink.
Simply because there's[br]fallacious reasoning
doesn't necessarily mean[br]that the conclusion
will be false.
It's interesting to note that this fallacy
is also the converse of[br]the fallacy of composition,
which says that it's fallacious
to infer that what is true[br]of the parts of something
is also true of the whole[br]those parts compose.
These fallacies are quite similar,
so you might want to check[br]out that video as well.
Anyway, watch out for this fallacy.
And remember that just[br]because water is wet,
doesn't mean that you can conclude
that water molecules are wet too.
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