Fallacies: Fallacy of Division

0:00(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.

0:22The 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.

0:43And 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.

1:05Premise (1): Whole A has[br]properties A, B, and C. Premise (2): P is a part of A.

1:13Conclusion: 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.

1:37So, 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.

1:55Conclusion: 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.

2:17So, 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.

2:39The 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.

3:00So, 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.

3:31It 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.

3:49Conclusion: 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.

4:13It'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.

4:33Anyway, 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. Subtitles by the Amara.org community