1 00:00:04,440 --> 00:00:07,675 Oh, hello. [SOUND] Welcome to the second unit of our 2 00:00:07,675 --> 00:00:10,986 course. In the first unit, you learned how to 3 00:00:10,986 --> 00:00:16,855 listen to what someone was saying, or read what they were writing, and find the 4 00:00:16,855 --> 00:00:21,369 argument in their words. You learned what arguments are, what 5 00:00:21,369 --> 00:00:24,530 their parts are, and what they're used for. 6 00:00:24,530 --> 00:00:29,598 In this second unit and the third unit that follows it, you're going to learn 7 00:00:29,598 --> 00:00:33,914 how to evaluate arguments. In the second unit, we're going to focus 8 00:00:33,914 --> 00:00:37,887 on deductive arguments. And in the third unit, you're going to 9 00:00:37,887 --> 00:00:42,682 focus on inductive arguments. So before we go any farther, I first need 10 00:00:42,682 --> 00:00:48,160 to explain the difference between deductive and inductive arguments. 11 00:00:48,160 --> 00:00:53,228 A deductive argument is an argument that's presented as being valid. 12 00:00:53,228 --> 00:00:57,103 Now, remember what it is for an argument to be valid. 13 00:00:57,103 --> 00:01:02,097 An argument is valid, just in case, there's no possible way for it's 14 00:01:02,097 --> 00:01:07,100 conclusion to be false, when all of it's premises are true. 15 00:01:07,100 --> 00:01:11,449 See, I just got a new pet clown fish, Nimo. 16 00:01:11,449 --> 00:01:16,432 Now, maybe you don't know much about clown fish anatomy. 17 00:01:16,432 --> 00:01:23,136 But I'm going to try to persuade you right now that clown fish have gills. 18 00:01:23,136 --> 00:01:30,294 Here's an argument that I can give you for the conclusion that clown fish have 19 00:01:30,294 --> 00:01:34,190 gills. Well, catfish have gills and goldfish 20 00:01:34,190 --> 00:01:40,960 have gills, and sharks have gills. Therefore, clown fish have gills. 21 00:01:40,960 --> 00:01:44,765 Now, is that argument valid? No, it's not. 22 00:01:44,765 --> 00:01:52,472 It's not valid because it's possible for the premises to be true even when the 23 00:01:52,472 --> 00:01:58,326 conclusion is false. It could be that catfish, and sharks, and 24 00:01:58,326 --> 00:02:04,120 goldfish all have gills even though clownfish don't. 25 00:02:04,120 --> 00:02:09,613 But now suppose I give you a different argument, for the conclusion that clown 26 00:02:09,613 --> 00:02:14,663 fish have gills. Here's how this different argument goes. 27 00:02:14,663 --> 00:02:19,167 All fish have gills, clown fish are a kind of fish. 28 00:02:19,167 --> 00:02:24,227 Therefore, clown fish have gills. Now, that argument is valid. 29 00:02:24,227 --> 00:02:30,356 There's no possible way for the premises of that argument to be true if the 30 00:02:30,356 --> 00:02:35,034 conclusion is false. I just gave you an example of a valid 31 00:02:35,034 --> 00:02:38,423 argument. But a valid argument is not necessarily a 32 00:02:38,423 --> 00:02:41,784 deductive argument. And the deductive argument is not 33 00:02:41,784 --> 00:02:45,996 necessarily a valid argument. A deductive argument is an argument 34 00:02:45,996 --> 00:02:50,489 that's presented as being valid. A deductive argument is successful only 35 00:02:50,489 --> 00:02:53,734 if it's valid. And in this second unit of the course, 36 00:02:53,734 --> 00:02:57,915 we're going to learn how to evaluate deductive arguments for validity. 37 00:02:57,915 --> 00:03:00,910 How to figure out whether or not they are valid. 38 00:03:00,910 --> 00:03:05,778 So, I just gave an example of an argument and we figured out that it was valid. 39 00:03:05,778 --> 00:03:10,520 We figured out that it was valid by noticing that there was no possible way 40 00:03:10,520 --> 00:03:14,140 for the premises to be true when the conclusion was false. 41 00:03:14,140 --> 00:03:19,171 So, if we could figure out that, that argument was valid, why do we need to do 42 00:03:19,171 --> 00:03:22,348 anything more for the second unit of the course? 43 00:03:22,348 --> 00:03:27,380 It looks like we can just look at an argument and figure out whether or not 44 00:03:27,380 --> 00:03:30,557 it's valid. What's there to learn in this second 45 00:03:30,557 --> 00:03:33,073 unit? Well, what we're going to do in this 46 00:03:33,073 --> 00:03:38,435 second unit is learn some rules that we can use for figuring out when particular 47 00:03:38,435 --> 00:03:43,711 arguments are valid. Now, why are these rules good to learn? 48 00:03:43,711 --> 00:03:48,231 There are three reasons why it's good to learn these rules. 49 00:03:48,231 --> 00:03:54,131 The first reason is that, sometimes using these rules can help you figure out 50 00:03:54,131 --> 00:04:00,107 whether a particular argument is valid. You see in some cases, it's not obvious 51 00:04:00,107 --> 00:04:04,704 just from looking at a particular argument that it is valid. 52 00:04:04,704 --> 00:04:10,321 Consider the following example. No fish have wings. 53 00:04:10,321 --> 00:04:16,572 All birds have wings. All animals with gills are fish. 54 00:04:16,572 --> 00:04:22,705 Therefore, no birds have gills. Is that argument valid? 55 00:04:22,705 --> 00:04:44,995 Take a moment to think about it. It's not obvious is it? 56 00:04:44,995 --> 00:04:54,460 Well, if you knew the rules for evaluating arguments for validity, 57 00:04:54,460 --> 00:05:00,580 you'd be able to use those rules to figure out that, that argument is valid. 58 00:05:00,580 --> 00:05:03,657 So, that's one reason why it's good to learn these rules. 59 00:05:03,657 --> 00:05:07,889 Sometimes they help you to figure out whether a particular argument is valid. 60 00:05:07,889 --> 00:05:11,296 But there's another reason why it's good to learn these rules. 61 00:05:11,296 --> 00:05:15,747 Even if they don't help you to figure out whether a particular argument is valid, 62 00:05:15,747 --> 00:05:19,430 they can help you to figure out why a particular argument is valid. 63 00:05:19,430 --> 00:05:26,158 So, in the example that we just considered about birds and fish, it turns 64 00:05:26,158 --> 00:05:31,111 out that there's a reason why that argument is valid. 65 00:05:31,111 --> 00:05:38,360 And the reason has to do with the meanings of the words, all and know. 66 00:05:38,360 --> 00:05:45,186 Some of the rules that we'll learn for evaluating the validity of arguments, are 67 00:05:45,186 --> 00:05:52,012 rules that tell us the meanings of the words all and know as they occur in that 68 00:05:52,012 --> 00:05:55,235 argument. And so those rules explain why the 69 00:05:55,235 --> 00:05:58,533 argument about the birds and the fish is valid. 70 00:05:58,533 --> 00:06:03,796 And they explain it in terms of the meaning of the words all and know that 71 00:06:03,796 --> 00:06:08,291 occur in that argument. So that's the second reason why it's good 72 00:06:08,291 --> 00:06:12,476 to learn these rules. Sometimes these rules help us to figure 73 00:06:12,476 --> 00:06:17,896 out why a particular argument is valid. There's a third reason why it's good to 74 00:06:17,896 --> 00:06:20,777 learn these rules for evaluating validity. 75 00:06:20,777 --> 00:06:25,648 And that is that, once we know these rules, evaluating the validity of a 76 00:06:25,648 --> 00:06:28,530 deductive argument becomes a lot more fun. 77 00:06:28,530 --> 00:06:35,146 To illustrate, here's some people trying to evaluate the validity of deductive 78 00:06:35,146 --> 00:06:42,322 arguments without using these rules. Now, here's some people trying to 79 00:06:42,322 --> 00:06:47,940 evaluate the validity of deductive arguments using these rules. 80 00:06:49,100 --> 00:06:53,371 See what I mean? The second group is obviously having more 81 00:06:53,371 --> 00:06:57,716 fun than the first. So, for those three reasons, it's good to 82 00:06:57,716 --> 00:07:01,766 learn these rules. They can help you figure out when an 83 00:07:01,766 --> 00:07:05,743 argument is valid. They can help you figure out why an 84 00:07:05,743 --> 00:07:09,573 argument is valid. And, it can be a lot fun to use them. 85 00:07:09,573 --> 00:07:11,856 So now, time to learn the rules.