WEBVTT 00:00:04.440 --> 00:00:10.459 Oh, hello. Welcome to the second unit of our course. In the first unit, you learned 00:00:10.459 --> 00:00:16.554 how to listen to what someone was saying, or read what they were writing, and find 00:00:16.554 --> 00:00:22.573 the argument in their words. You learned what arguments are, what their parts are, 00:00:22.573 --> 00:00:28.297 and what they're used for. In this second unit and the third unit that follows it, 00:00:28.297 --> 00:00:33.503 you're going to learn how to evaluate arguments. In the second unit, we're going 00:00:33.092 --> 00:00:38.503 to focus on deductive arguments. And in the third unit, you're going to focus on 00:00:38.503 --> 00:00:43.709 inductive arguments. So before we go any farther, I first need to explain the 00:00:43.709 --> 00:00:50.172 difference between deductive and inductive arguments. A deductive argument is an 00:00:50.172 --> 00:00:56.209 argument that's presented as being valid. Now, remember what it is for an argument 00:00:56.209 --> 00:01:02.097 to be valid. An argument is valid, just in case, there's no possible way for it's 00:01:02.097 --> 00:01:09.002 conclusion to be false, when all of it's premises are true. See, I just got a new 00:01:09.002 --> 00:01:16.794 pet clown fish, Nimo. Now, maybe you don't know much about clown fish anatomy. But 00:01:16.794 --> 00:01:24.043 I'm going to try to persuade you right now that clown fish have gills. Here's an 00:01:24.043 --> 00:01:30.838 argument that I can give you for the conclusion that clown fish have gills. 00:01:30.838 --> 00:01:38.086 Well, catfish have gills and goldfish have gills, and sharks have gills. Therefore, 00:01:38.086 --> 00:01:45.643 clown fish have gills. Now, is that argument valid? No, it's not. It's not 00:01:45.643 --> 00:01:52.472 valid because it's possible for the premises to be true even when the 00:01:52.472 --> 00:02:00.082 conclusion is false. It could be that catfish, and sharks, and goldfish all have 00:02:00.082 --> 00:02:07.500 gills even though clownfish don't. But now suppose I give you a different argument, 00:02:07.500 --> 00:02:14.212 for the conclusion that clown fish have gills. Here's how this different argument 00:02:14.212 --> 00:02:21.509 goes. All fish have gills, clown fish are a kind of fish. Therefore, clown fish have 00:02:21.509 --> 00:02:27.534 gills. Now, that argument is valid. There's no possible way for the premises 00:02:27.534 --> 00:02:33.501 of that argument to be true if the conclusion is false. I just gave you an 00:02:33.501 --> 00:02:39.057 example of a valid argument. But a valid argument is not necessarily a deductive 00:02:39.057 --> 00:02:43.874 argument. And the deductive argument is not necessarily a valid argument. A 00:02:43.874 --> 00:02:48.742 deductive argument is an argument t hat's presented as being valid. A deductive 00:02:48.742 --> 00:02:53.734 argument is successful only if it's valid. And in this second unit of the course, 00:02:53.734 --> 00:02:58.352 we're going to learn how to evaluate deductive arguments for validity. How to 00:02:58.352 --> 00:03:03.469 figure out whether or not they are valid. So, I just gave an example of an argument 00:03:03.469 --> 00:03:08.586 and we figured out that it was valid. We figured out that it was valid by noticing 00:03:08.586 --> 00:03:13.515 that there was no possible way for the premises to be true when the conclusion 00:03:13.515 --> 00:03:18.443 was false. So, if we could figure out that, that argument was valid, why do we 00:03:18.443 --> 00:03:23.739 need to do anything more for the second unit of the course? It looks like we can 00:03:23.739 --> 00:03:29.167 just look at an argument and figure out whether or not it's valid. What's there to 00:03:29.167 --> 00:03:34.066 learn in this second unit? Well, what we're going to do in this second unit is 00:03:34.066 --> 00:03:39.362 learn some rules that we can use for figuring out when particular arguments are 00:03:39.362 --> 00:03:46.239 valid. Now, why are these rules good to learn? There are three reasons why it's 00:03:46.239 --> 00:03:51.833 good to learn these rules. The first reason is that, sometimes using these 00:03:51.833 --> 00:03:57.962 rules can help you figure out whether a particular argument is valid. You see in 00:03:57.962 --> 00:04:04.015 some cases, it's not obvious just from looking at a particular argument that it 00:04:04.015 --> 00:04:12.797 is valid. Consider the following example. No fish have wings. All birds have wings. 00:04:12.797 --> 00:04:20.936 All animals with gills are fish. Therefore, no birds have gills. Is that 00:04:20.936 --> 00:04:46.121 argument valid? Take a moment to think about it. It's not obvious is it? Well, if 00:04:46.121 --> 00:04:55.770 you knew the rules for evaluating arguments for validity, you'd be able to 00:04:55.770 --> 00:05:01.734 use those rules to figure out that, that argument is valid. So, that's one reason 00:05:01.734 --> 00:05:05.745 why it's good to learn these rules. Sometimes they help you to figure out 00:05:05.745 --> 00:05:10.142 whether a particular argument is valid. But there's another reason why it's good 00:05:10.142 --> 00:05:14.154 to learn these rules. Even if they don't help you to figure out whether a 00:05:14.154 --> 00:05:18.440 particular argument is valid, they can help you to figure out why a particular 00:05:18.440 --> 00:05:25.317 argument is valid. So, in the example that we just considered about birds and fish, 00:05:25.317 --> 00:05:32.887 it turns out that there's a reason why that argument is valid. And the reason has 00:05:32.887 --> 00:05:40.834 to do wi th the meanings of the words, all and know. Some of the rules that we'll 00:05:40.834 --> 00:05:47.148 learn for evaluating the validity of arguments, are rules that tell us the 00:05:47.148 --> 00:05:54.113 meanings of the words "all" and "no" as they occur in that argument. And so those rules 00:05:54.113 --> 00:05:59.937 explain why the argument about the birds and the fish is valid. And they explain it 00:05:59.937 --> 00:06:05.615 in terms of the meaning of the words "all" and "no" that occur in that argument. So 00:06:05.615 --> 00:06:11.241 that's the second reason why it's good to learn these rules. Sometimes these rules 00:06:11.241 --> 00:06:16.729 help us to figure out why a particular argument is valid. There's a third reason 00:06:16.729 --> 00:06:21.943 why it's good to learn these rules for evaluating validity. And that is that, 00:06:21.943 --> 00:06:27.638 once we know these rules, evaluating the validity of a deductive argument becomes a 00:06:27.638 --> 00:06:34.298 lot more fun. To illustrate, here's some people trying to evaluate the validity of 00:06:34.298 --> 00:06:42.322 deductive arguments without using these rules. Now, here's some people trying to 00:06:42.322 --> 00:06:49.910 evaluate the validity of deductive arguments using these rules. See what I 00:06:49.910 --> 00:06:55.727 mean? The second group is obviously having more fun than the first. So, for those 00:06:55.727 --> 00:07:01.545 three reasons, it's good to learn these rules. They can help you figure out when 00:07:01.545 --> 00:07:07.363 an argument is valid. They can help you figure out why an argument is valid. And, 00:07:07.363 --> 00:07:11.856 it can be a lot fun to use them. So now, time to learn the rules.