WEBVTT 00:00:00.000 --> 00:00:06.511 (intro music) 00:00:06.511 --> 00:00:08.482 Hello, I'm Matthew Harris, 00:00:08.482 --> 00:00:10.643 and I'm a philosophy grad student at Duke University. 00:00:10.643 --> 00:00:12.771 And today, I'll be discussing the formal fallacy 00:00:12.771 --> 00:00:14.105 of affirming the consequent, 00:00:14.105 --> 00:00:15.928 and why you sometimes cannot conclude 00:00:15.928 --> 00:00:18.193 that you should bathe in a tub of peanut butter. 00:00:18.193 --> 00:00:19.691 Affirming the consequent occurs 00:00:19.691 --> 00:00:21.409 when someone tries to infer the truth 00:00:21.409 --> 00:00:23.334 of the antecedent of a conditional statement 00:00:23.334 --> 00:00:27.086 from the truth of the conditional and its consequent. 00:00:27.086 --> 00:00:28.989 But let's see what this means in more detail. 00:00:28.989 --> 00:00:30.928 There are two kinds of logical fallacies: 00:00:30.928 --> 00:00:32.412 formal and informal. 00:00:32.412 --> 00:00:35.013 Both kinds are defective argumentative patterns. 00:00:35.013 --> 00:00:36.570 First, we have informal fallacies, 00:00:36.570 --> 00:00:38.695 which lack support for the conclusion 00:00:38.695 --> 00:00:40.669 because of a flaw in its content. 00:00:40.669 --> 00:00:42.689 We also have formal fallacies, 00:00:42.689 --> 00:00:45.091 which all have in common with affirming the consequent 00:00:45.091 --> 00:00:47.866 that they have defects in the forms of the argument 00:00:47.866 --> 00:00:49.654 and that they are invalid. 00:00:49.654 --> 00:00:52.347 Just to be clear, let's go over a few more definitions. 00:00:52.347 --> 00:00:54.728 We make conditional statements all the time. 00:00:54.728 --> 00:00:56.225 They're generally easy to spot 00:00:56.225 --> 00:00:57.769 because they usually are of the form 00:00:57.769 --> 00:00:59.987 "if P, then Q." 00:00:59.987 --> 00:01:02.251 Here, "P" is the antecedent. 00:01:02.251 --> 00:01:03.573 An easy way to spot antecedents 00:01:03.573 --> 00:01:04.828 is to remember that they typically 00:01:04.828 --> 00:01:06.824 come after the word "if," whether or not they're 00:01:06.824 --> 00:01:09.052 at the beginning, middle or end of sentences. 00:01:09.052 --> 00:01:10.632 If you need help remembering that, 00:01:10.632 --> 00:01:12.130 just remember that the antecedent comes 00:01:12.130 --> 00:01:14.068 before the other logically, 00:01:14.068 --> 00:01:17.087 and that it sounds a lot like "ancestor." 00:01:17.087 --> 00:01:19.496 The consequent of the conditional 00:01:19.496 --> 00:01:20.473 is the part that typically follows 00:01:20.473 --> 00:01:22.249 after the word "then." 00:01:22.249 --> 00:01:23.330 It should be easy to remember 00:01:23.330 --> 00:01:25.256 because it sounds like "consequence" 00:01:25.256 --> 00:01:27.199 and basically is just that. 00:01:28.369 --> 00:01:31.020 So let's take the following conditionals for examples. 00:01:31.020 --> 00:01:32.296 Suppose someone tells you the following 00:01:32.296 --> 00:01:34.480 true conditionals and statement: 00:01:34.480 --> 00:01:36.266 "If the neighbors ate Susan's parrot, 00:01:36.266 --> 00:01:37.927 "then Susan is angry," 00:01:37.927 --> 00:01:40.401 and "Susan is angry." 00:01:40.401 --> 00:01:42.339 Just because it is true that if the neighbors 00:01:42.339 --> 00:01:45.544 had eaten the parrot, then she would have been angry, 00:01:45.544 --> 00:01:48.282 and it is also true that she is angry, 00:01:48.282 --> 00:01:50.059 does not mean that she's angry 00:01:50.059 --> 00:01:53.204 because they ate her parrot. 00:01:53.204 --> 00:01:54.644 Perhaps she's mad because her parrot 00:01:54.644 --> 00:01:56.503 isn't very interesting. 00:01:56.503 --> 00:01:58.244 Or maybe she's angry that it doesn't know 00:01:58.244 --> 00:01:59.706 how to use the toy car that she spent 00:01:59.706 --> 00:02:01.657 all afternoon building for it. 00:02:01.657 --> 00:02:04.083 Nevertheless, it does not follow from the conjunction 00:02:04.083 --> 00:02:06.602 of the true conditional and the true consequent 00:02:06.602 --> 00:02:09.711 that the antecedent is true. 00:02:09.711 --> 00:02:11.994 Let's look at a few more examples: 00:02:11.994 --> 00:02:13.898 "If Tom has a good reason to complain, 00:02:13.898 --> 00:02:16.894 "then Tom will complain tomorrow." 00:02:16.894 --> 00:02:19.795 Now, maybe you know Tom well, 00:02:19.795 --> 00:02:21.757 so you know that this is true. 00:02:21.757 --> 00:02:23.277 Maybe you even know that it's true 00:02:23.277 --> 00:02:25.496 that he will complain tomorrow. 00:02:25.496 --> 00:02:26.831 But it would not follow that Tom 00:02:26.831 --> 00:02:29.060 has a good reason to complain. 00:02:29.060 --> 00:02:30.417 Maybe he just doesn't know 00:02:30.417 --> 00:02:32.612 any better way to get attention. 00:02:32.612 --> 00:02:34.993 Now, let's take a look at one more example. 00:02:34.993 --> 00:02:38.151 Consider this conditional and the assertion: 00:02:38.151 --> 00:02:40.054 "If you are allergic to peanut butter, 00:02:40.054 --> 00:02:41.633 "then it would be a bad idea 00:02:41.633 --> 00:02:43.838 "to bathe in a tub of peanut butter," 00:02:43.838 --> 00:02:46.088 and "it is a bad idea to bathe 00:02:46.088 --> 00:02:48.476 "in a tub of peanut butter; 00:02:48.476 --> 00:02:51.076 "therefore, you are allergic to peanut butter." 00:02:52.265 --> 00:02:53.478 Just because it is true that it would be 00:02:53.478 --> 00:02:55.963 a bad idea to bathe in a tub of peanut butter 00:02:55.963 --> 00:02:57.402 if you are allergic, 00:02:57.402 --> 00:02:59.260 and it is also true that it is a bad idea 00:02:59.260 --> 00:03:01.918 to bathe in a tub of peanut butter in general, 00:03:01.918 --> 00:03:04.739 does not mean that you are allergic to peanut butter. 00:03:04.739 --> 00:03:06.458 If you were to conclude this, 00:03:06.458 --> 00:03:07.884 then you would be committing the fallacy 00:03:07.884 --> 00:03:09.835 of affirming the consequent. 00:03:09.835 --> 00:03:10.857 So that's the formal fallacy 00:03:10.857 --> 00:03:12.158 of affirming the consequent, 00:03:12.158 --> 00:03:14.919 and a few examples that you could use in the future.