WEBVTT 00:00:00.506 --> 00:00:06.606 [ Music ] 00:00:07.106 --> 00:00:11.166 >> Hello. I'm Paul Henne, and I'm a philosophy gradate student at Duke University. 00:00:11.646 --> 00:00:16.366 And in this video, I'm going to introduce you to formal and informal fallacies. 00:00:16.986 --> 00:00:19.356 First, consider the following arguments. 00:00:19.886 --> 00:00:24.406 Premise one: if someone is allergic to peanuts, then she doesn't eat peanut butter. 00:00:24.856 --> 00:00:27.266 Premise two: Jane doesn't eat peanut butter. 00:00:27.606 --> 00:00:30.526 Conclusion: therefore Jane is allergic to peanuts. 00:00:31.446 --> 00:00:32.596 Consider this other argument. 00:00:33.206 --> 00:00:35.536 Premise one: a feather is light. 00:00:36.146 --> 00:00:38.646 Premise two: what's light cannot be dark. 00:00:39.026 --> 00:00:42.466 Conclusion: therefore a feather cannot be dark. 00:00:43.556 --> 00:00:46.766 These arguments probably seemed a bit off to you, and you're right. 00:00:47.236 --> 00:00:51.116 Each argument contains a fallacy, though each has a different kind. 00:00:51.866 --> 00:00:57.106 A fallacy, as we've seen in other wireless philosophy videos, is a defect in reasoning. 00:00:57.516 --> 00:01:01.076 This defect can be unintentional or intentional. 00:01:01.486 --> 00:01:05.156 That is, I could simply make a mistake in my reasoning. 00:01:05.456 --> 00:01:09.296 Or I could want to trick you into believing something by using deceptive reasoning. 00:01:10.366 --> 00:01:14.276 Either way, it's probably a good idea that we are able to understand 00:01:14.276 --> 00:01:16.856 and identify distinct fallacy types 00:01:17.096 --> 00:01:20.846 so that we can adequately correct any defects in our reasoning. 00:01:22.006 --> 00:01:27.456 But before we talk about fallacy types, let's recall what a valid argument is. 00:01:28.136 --> 00:01:29.206 Consider this argument. 00:01:29.546 --> 00:01:34.706 Premise one: if someone is a philosopher, then she publishes articles on philosophy. 00:01:34.866 --> 00:01:41.026 So Tamar Gendler is a philosopher, therefore, Tamar Gendler publishes articles in philosophy. 00:01:42.136 --> 00:01:43.166 This is a valid argument. 00:01:43.806 --> 00:01:47.936 Actually, although it isn't bearing on this argument's validity, Gendler has published 00:01:47.936 --> 00:01:50.356 over 20 articles, among her other works. 00:01:51.046 --> 00:01:56.636 This form of argument is called modus ponens, and it can be represented as the following. 00:01:57.246 --> 00:02:03.886 Premise one: if X, then Y. Premise two: X. Conclusion: therefore Y. 00:02:04.526 --> 00:02:07.286 And this is a valid argument form. 00:02:07.676 --> 00:02:11.016 The conclusion necessarily follows from the premises. 00:02:11.706 --> 00:02:16.086 Remember though, this valid form does not mean that the premises are true. 00:02:16.586 --> 00:02:21.246 It would still be a valid argument if Tamar Gendler had decided to become a train conductor, 00:02:21.246 --> 00:02:23.006 for instance, instead of a philosopher. 00:02:23.226 --> 00:02:28.386 See the videos on soundness and validity if you have questions about this. 00:02:30.386 --> 00:02:33.016 We can also have an invalid argument form. 00:02:33.586 --> 00:02:35.226 Let's return to our first argument. 00:02:36.246 --> 00:02:41.546 This argument, as you might have suspected, contains a fallacy, a formal fallacy. 00:02:42.036 --> 00:02:48.226 And a formal fallacy is exactly what it sounds like: a defect in the form of the argument. 00:02:49.026 --> 00:02:53.256 In other words, there are certain forms of arguments that are invalid. 00:02:54.836 --> 00:02:59.746 This invalid argument form is a formal fallacy called affirming the consequent. 00:03:00.656 --> 00:03:05.506 This fallacy is apparent if you consider that Jane just might not like peanut butter. 00:03:05.996 --> 00:03:09.826 So simply because Jane doesn't like peanut butter, she doesn't eat it. 00:03:10.536 --> 00:03:15.976 Given just the fact that she doesn't eat it and the condition in premise one, we cannot conclude 00:03:15.976 --> 00:03:17.666 that Jane is allergic to peanut butter. 00:03:18.766 --> 00:03:21.846 This form of fallacy can be represented in the following way. 00:03:22.746 --> 00:03:31.806 If X, then Y. Y, therefore X. And this form of fallacy is one that you don't want to use. 00:03:32.056 --> 00:03:35.666 It's actually one of the fallacies frequently tested on the LSAT. 00:03:36.136 --> 00:03:40.156 So if you're interested and want more practice in identifying this fallacy, 00:03:40.616 --> 00:03:43.036 check out the video on affirming the consequent. 00:03:44.466 --> 00:03:48.726 We will talk more about this and other formal fallacies in upcoming videos. 00:03:48.986 --> 00:03:53.746 But for now, we need to see that formal fallacies are fallacies 00:03:54.126 --> 00:03:56.406 because of the poor form of the argument. 00:03:57.496 --> 00:04:00.746 Any argument of these forms will be invalid. 00:04:01.546 --> 00:04:05.166 So anything can be plugged into these bad argument forms 00:04:05.556 --> 00:04:06.906 and the argument would be invalid. 00:04:07.376 --> 00:04:10.516 For instance, if meow, then splat. 00:04:11.076 --> 00:04:13.936 Splat, therefore meow, is invalid. 00:04:15.626 --> 00:04:21.946 So now that we have a general understanding of formal fallacies, what about informal ones? 00:04:21.946 --> 00:04:25.486 Lt's look at our other example from the beginning of this video. 00:04:26.626 --> 00:04:31.686 Notice that this fallacy arises out of the content of the argument, not out of the form. 00:04:32.156 --> 00:04:35.216 The form may at first glance seem valid. 00:04:35.456 --> 00:04:38.246 That is if we ignored the meaning of the content. 00:04:39.106 --> 00:04:44.486 Consider that we can use a seemingly similar argument form to make a valid argument. 00:04:45.016 --> 00:04:48.106 For instance, premise one: rain is wet. 00:04:48.726 --> 00:04:51.156 Premise two: what's wet cannot be dry. 00:04:51.616 --> 00:04:54.546 Conclusion: therefore rain cannot be dry. 00:04:54.916 --> 00:05:00.636 And here we have a valid argument that seems to have a similar form as the fallacious one, 00:05:01.266 --> 00:05:04.216 and one that lacks defects in its content. 00:05:04.496 --> 00:05:10.426 But the content of our fallacious argument, given the two meanings of light -- 00:05:10.916 --> 00:05:14.976 lightweight and light color -- yields some problems. 00:05:15.166 --> 00:05:19.636 That is, if we understand the meaning of the terms -- the two meanings of light, 00:05:19.636 --> 00:05:24.586 for instance -- we realize that the argument actually has an invalid argument form. 00:05:25.686 --> 00:05:29.446 Initially, without considering the meaning of the content, it looks like the form 00:05:29.446 --> 00:05:40.076 of the argument is something like, all X are Y. What is Y cannot be Z. Therefore no X are Z. 00:05:40.856 --> 00:05:45.636 But actually, given the shifting meaning of light, the form is more like, 00:05:46.316 --> 00:05:56.346 all X are Y. What is W cannot be Z. Therefore no X are Z. This fallacy is called equivocation. 00:05:57.086 --> 00:06:00.126 And see the video on this if you'd like to know more about it. 00:06:01.546 --> 00:06:07.066 The only way we can understand this defect is by examining the content of the argument. 00:06:07.586 --> 00:06:10.896 If we just looked at the form without understanding the content, 00:06:11.296 --> 00:06:13.576 then we could not detect this fallacy. 00:06:14.876 --> 00:06:18.986 So as we now know, informal fallacies occur because of problems 00:06:18.986 --> 00:06:20.956 with the content of the arguments. 00:06:22.126 --> 00:06:27.106 So an argument might have a seemingly valid form but commit an informal fallacy 00:06:27.106 --> 00:06:29.006 because of a defect in its content. 00:06:29.686 --> 00:06:32.086 Hence, an argument might have true premises 00:06:32.086 --> 00:06:36.826 and a seemingly valid form, yet also commit a fallacy. 00:06:38.286 --> 00:06:42.866 To reiterate, a formal fallacy means that the argument has a defect in its form, 00:06:43.396 --> 00:06:47.216 while an informal fallacy has a defect in the argument's content. 00:06:47.536 --> 00:06:50.236 Which might also yield a defect in its form. 00:06:50.806 --> 00:06:54.506 So now that's the difference between formal and informal fallacies.