[Script Info]
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[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:05.84,Default,,0000,0000,0000,,(intro music)
Dialogue: 0,0:00:05.84,0:00:07.92,Default,,0000,0000,0000,,Hello, I'm Paul Henne
Dialogue: 0,0:00:07.92,0:00:11.09,Default,,0000,0000,0000,,and I'm a philosophy graduate\Nstudent at Duke University.
Dialogue: 0,0:00:11.09,0:00:14.40,Default,,0000,0000,0000,,And in this video I'm\Ngoing to discuss validity,
Dialogue: 0,0:00:14.40,0:00:18.59,Default,,0000,0000,0000,,an important tool for\Nevaluating deductive arguments.
Dialogue: 0,0:00:18.96,0:00:20.79,Default,,0000,0000,0000,,You've probably heard someone say
Dialogue: 0,0:00:20.79,0:00:22.40,Default,,0000,0000,0000,,"that's a valid point," or
Dialogue: 0,0:00:22.40,0:00:23.83,Default,,0000,0000,0000,,maybe in an argument you've heard a friend
Dialogue: 0,0:00:23.83,0:00:26.58,Default,,0000,0000,0000,,say something like\N"that's valid, but..."
Dialogue: 0,0:00:26.58,0:00:29.14,Default,,0000,0000,0000,,In these everyday uses of the term
Dialogue: 0,0:00:29.14,0:00:31.70,Default,,0000,0000,0000,,"valid" or "validity,"
Dialogue: 0,0:00:31.70,0:00:34.27,Default,,0000,0000,0000,,people often mean to\Nconvey something like
Dialogue: 0,0:00:34.27,0:00:35.93,Default,,0000,0000,0000,,"that's a good point," or
Dialogue: 0,0:00:35.93,0:00:37.70,Default,,0000,0000,0000,,"that statement's true."
Dialogue: 0,0:00:37.70,0:00:38.81,Default,,0000,0000,0000,,But I won't be talking,
Dialogue: 0,0:00:38.81,0:00:40.27,Default,,0000,0000,0000,,in this video at least,
Dialogue: 0,0:00:40.27,0:00:42.29,Default,,0000,0000,0000,,about those usages.
Dialogue: 0,0:00:42.29,0:00:44.90,Default,,0000,0000,0000,,Instead, I'll be discussing the technical
Dialogue: 0,0:00:44.90,0:00:47.64,Default,,0000,0000,0000,,philosophical notion of validity,
Dialogue: 0,0:00:47.64,0:00:50.10,Default,,0000,0000,0000,,as in "a valid argument."
Dialogue: 0,0:00:50.10,0:00:53.54,Default,,0000,0000,0000,,You already know that an\Nargument is a set of statements,
Dialogue: 0,0:00:53.54,0:00:56.38,Default,,0000,0000,0000,,and that one or more of these statements
Dialogue: 0,0:00:56.38,0:00:58.20,Default,,0000,0000,0000,,is offered in support
Dialogue: 0,0:00:58.20,0:01:00.01,Default,,0000,0000,0000,,of some other statements.
Dialogue: 0,0:01:00.78,0:01:03.36,Default,,0000,0000,0000,,The statements doing the supporting
Dialogue: 0,0:01:03.36,0:01:05.44,Default,,0000,0000,0000,,are called "premises,"
Dialogue: 0,0:01:05.44,0:01:07.56,Default,,0000,0000,0000,,and the statements being supported
Dialogue: 0,0:01:07.56,0:01:09.53,Default,,0000,0000,0000,,are called "conclusions."
Dialogue: 0,0:01:10.06,0:01:12.60,Default,,0000,0000,0000,,Validity, in the sense\Nthat I'm talking about it,
Dialogue: 0,0:01:12.60,0:01:15.32,Default,,0000,0000,0000,,applies to deductive arguments.
Dialogue: 0,0:01:15.32,0:01:18.73,Default,,0000,0000,0000,,So an argument is valid or invalid.
Dialogue: 0,0:01:18.73,0:01:21.10,Default,,0000,0000,0000,,Validity, then, isn't a\Nproperty of statements
Dialogue: 0,0:01:21.10,0:01:22.48,Default,,0000,0000,0000,,or anything of the like.
Dialogue: 0,0:01:22.48,0:01:25.89,Default,,0000,0000,0000,,So, what exactly is a valid argument then?
Dialogue: 0,0:01:25.89,0:01:28.43,Default,,0000,0000,0000,,Well, suppose that you make\Nthe following argument,
Dialogue: 0,0:01:28.43,0:01:31.55,Default,,0000,0000,0000,,and here I'll use "P"s\Nto stand for "premises"
Dialogue: 0,0:01:31.55,0:01:34.24,Default,,0000,0000,0000,,and I'll use a "C" to\Nstand for the conclusion.
Dialogue: 0,0:01:34.95,0:01:37.92,Default,,0000,0000,0000,,(P1): All humans are mortal.
Dialogue: 0,0:01:37.92,0:01:41.06,Default,,0000,0000,0000,,(P2): Iris Murdoch is a human.
Dialogue: 0,0:01:41.06,0:01:44.100,Default,,0000,0000,0000,,(C): Therefore, Iris Murdoch is mortal.
Dialogue: 0,0:01:46.06,0:01:49.04,Default,,0000,0000,0000,,Suppose that I say that\Nyour argument is valid.
Dialogue: 0,0:01:49.04,0:01:52.08,Default,,0000,0000,0000,,Do I mean to say that\Nyour argument is good?
Dialogue: 0,0:01:52.08,0:01:54.24,Default,,0000,0000,0000,,Do I mean to say that your conclusion,
Dialogue: 0,0:01:54.24,0:01:57.70,Default,,0000,0000,0000,,or that all of the premises\Nand the conclusion, are true?
Dialogue: 0,0:01:57.70,0:02:00.14,Default,,0000,0000,0000,,While this might sound\Nlike what I'm saying,
Dialogue: 0,0:02:00.14,0:02:01.80,Default,,0000,0000,0000,,validity has nothing to do
Dialogue: 0,0:02:01.80,0:02:03.84,Default,,0000,0000,0000,,with the truth of the conclusion
Dialogue: 0,0:02:03.84,0:02:07.75,Default,,0000,0000,0000,,or with how good the\Nargument is in general.
Dialogue: 0,0:02:08.32,0:02:09.88,Default,,0000,0000,0000,,So, let's define it.
Dialogue: 0,0:02:10.19,0:02:14.40,Default,,0000,0000,0000,,An argument is valid if and only if
Dialogue: 0,0:02:14.40,0:02:16.46,Default,,0000,0000,0000,,the truth of its premises
Dialogue: 0,0:02:16.46,0:02:20.17,Default,,0000,0000,0000,,guarantees the truth of its conclusion.
Dialogue: 0,0:02:20.17,0:02:23.96,Default,,0000,0000,0000,,That is, validity is a\Nproperty of arguments,
Dialogue: 0,0:02:23.96,0:02:28.07,Default,,0000,0000,0000,,such that if the premises\Nof the arguments are true,
Dialogue: 0,0:02:28.07,0:02:31.45,Default,,0000,0000,0000,,then the conclusion must be true.
Dialogue: 0,0:02:32.15,0:02:35.91,Default,,0000,0000,0000,,So it's impossible for a valid argument
Dialogue: 0,0:02:35.91,0:02:38.60,Default,,0000,0000,0000,,to have all true premises
Dialogue: 0,0:02:38.60,0:02:41.82,Default,,0000,0000,0000,,unless the conclusion is also true.
Dialogue: 0,0:02:42.44,0:02:45.16,Default,,0000,0000,0000,,When an argument is valid in this sense,
Dialogue: 0,0:02:45.16,0:02:49.60,Default,,0000,0000,0000,,we say that the premises\Nentail the conclusion.
Dialogue: 0,0:02:50.04,0:02:52.89,Default,,0000,0000,0000,,So, let's back up for a second.
Dialogue: 0,0:02:52.89,0:02:55.52,Default,,0000,0000,0000,,An argument is composed of statements.
Dialogue: 0,0:02:55.52,0:02:57.44,Default,,0000,0000,0000,,Statements can be true or false,
Dialogue: 0,0:02:57.44,0:03:00.52,Default,,0000,0000,0000,,like the statement "this\Nsquare is orange."
Dialogue: 0,0:03:00.52,0:03:03.13,Default,,0000,0000,0000,,Arguments cannot be true or false.
Dialogue: 0,0:03:03.15,0:03:06.43,Default,,0000,0000,0000,,They can, however, be valid or invalid,
Dialogue: 0,0:03:06.43,0:03:08.24,Default,,0000,0000,0000,,as well as other things.
Dialogue: 0,0:03:08.24,0:03:11.02,Default,,0000,0000,0000,,And, if an argument is valid,
Dialogue: 0,0:03:11.02,0:03:13.12,Default,,0000,0000,0000,,then if its premises are true,
Dialogue: 0,0:03:13.12,0:03:15.24,Default,,0000,0000,0000,,its conclusion is true.
Dialogue: 0,0:03:16.17,0:03:19.40,Default,,0000,0000,0000,,Notice that I have not\Nsaid that a valid argument
Dialogue: 0,0:03:19.40,0:03:21.82,Default,,0000,0000,0000,,has true or false premises
Dialogue: 0,0:03:21.82,0:03:23.98,Default,,0000,0000,0000,,or a true and false conclusion.
Dialogue: 0,0:03:23.98,0:03:26.56,Default,,0000,0000,0000,,I have said something conditional.
Dialogue: 0,0:03:26.56,0:03:29.50,Default,,0000,0000,0000,,That is, if the argument is valid,
Dialogue: 0,0:03:29.50,0:03:31.36,Default,,0000,0000,0000,,then the truth of its conclusion
Dialogue: 0,0:03:31.36,0:03:34.25,Default,,0000,0000,0000,,follows from the truth of its premises.
Dialogue: 0,0:03:34.76,0:03:38.11,Default,,0000,0000,0000,,Conversely, if the truth of the premises
Dialogue: 0,0:03:38.11,0:03:39.83,Default,,0000,0000,0000,,entails the conclusion,
Dialogue: 0,0:03:39.83,0:03:42.64,Default,,0000,0000,0000,,then the argument is valid.
Dialogue: 0,0:03:43.77,0:03:45.17,Default,,0000,0000,0000,,Now, this all sounds very abstract,
Dialogue: 0,0:03:45.17,0:03:46.82,Default,,0000,0000,0000,,so let's return to some examples.
Dialogue: 0,0:03:46.82,0:03:48.68,Default,,0000,0000,0000,,Let's look at our previous example.
Dialogue: 0,0:03:48.68,0:03:49.84,Default,,0000,0000,0000,,I have said that the argument
Dialogue: 0,0:03:49.84,0:03:53.31,Default,,0000,0000,0000,,about the British philosopher,\NIris Murdoch is valid.
Dialogue: 0,0:03:53.31,0:03:54.73,Default,,0000,0000,0000,,Am I right?
Dialogue: 0,0:03:54.73,0:03:55.31,Default,,0000,0000,0000,,Yes!
Dialogue: 0,0:03:55.31,0:03:57.87,Default,,0000,0000,0000,,If the premises of the argument are true,
Dialogue: 0,0:03:57.87,0:04:00.74,Default,,0000,0000,0000,,then the conclusion must\Nbe true, in this case.
Dialogue: 0,0:04:01.29,0:04:02.60,Default,,0000,0000,0000,,Remember, it doesn't matter
Dialogue: 0,0:04:02.60,0:04:05.09,Default,,0000,0000,0000,,if our premises are true or false.
Dialogue: 0,0:04:05.09,0:04:06.50,Default,,0000,0000,0000,,Consider, for example, an argument
Dialogue: 0,0:04:06.50,0:04:08.84,Default,,0000,0000,0000,,with all false premises in it.
Dialogue: 0,0:04:08.84,0:04:12.53,Default,,0000,0000,0000,,(P1): All humans are immortal.
Dialogue: 0,0:04:12.53,0:04:15.72,Default,,0000,0000,0000,,Premise (2): Iris Murdoch is a human.
Dialogue: 0,0:04:15.72,0:04:19.90,Default,,0000,0000,0000,,Conclusion: Therefore,\NIris Murdoch is immortal.
Dialogue: 0,0:04:20.81,0:04:23.24,Default,,0000,0000,0000,,This argument is also valid,
Dialogue: 0,0:04:23.24,0:04:24.51,Default,,0000,0000,0000,,just like the first argument.
Dialogue: 0,0:04:24.51,0:04:26.68,Default,,0000,0000,0000,,The truth of the premises entails
Dialogue: 0,0:04:26.68,0:04:29.10,Default,,0000,0000,0000,,the truth of the conclusion, right?
Dialogue: 0,0:04:29.10,0:04:32.84,Default,,0000,0000,0000,,If it is the case that\Nall humans are immortal,
Dialogue: 0,0:04:32.84,0:04:37.15,Default,,0000,0000,0000,,and it is the case that Iris\NMurdoch is one of these humans,
Dialogue: 0,0:04:37.15,0:04:38.89,Default,,0000,0000,0000,,then it's necessarily the case
Dialogue: 0,0:04:38.89,0:04:41.43,Default,,0000,0000,0000,,that Iris Murdoch is immortal.
Dialogue: 0,0:04:41.43,0:04:43.53,Default,,0000,0000,0000,,Let's try an example with premises
Dialogue: 0,0:04:43.53,0:04:45.68,Default,,0000,0000,0000,,of which we don't know the truth.
Dialogue: 0,0:04:46.02,0:04:49.51,Default,,0000,0000,0000,,(P1): All aliens speak English.
Dialogue: 0,0:04:49.51,0:04:52.58,Default,,0000,0000,0000,,(P2): Splock is an alien.
Dialogue: 0,0:04:52.58,0:04:56.46,Default,,0000,0000,0000,,Conclusion: Therefore,\NSplock speaks English.
Dialogue: 0,0:04:56.46,0:04:58.43,Default,,0000,0000,0000,,We don't know if there are aliens,
Dialogue: 0,0:04:58.43,0:05:00.32,Default,,0000,0000,0000,,let alone ones that can speak at all.
Dialogue: 0,0:05:00.32,0:05:02.37,Default,,0000,0000,0000,,We don't know if they speak English.
Dialogue: 0,0:05:02.37,0:05:03.17,Default,,0000,0000,0000,,It could be the case,
Dialogue: 0,0:05:03.17,0:05:04.69,Default,,0000,0000,0000,,or it couldn't be the case.
Dialogue: 0,0:05:04.69,0:05:07.29,Default,,0000,0000,0000,,But this argument, nonetheless, is valid.
Dialogue: 0,0:05:07.29,0:05:10.03,Default,,0000,0000,0000,,If premise one and two are true,
Dialogue: 0,0:05:10.03,0:05:12.66,Default,,0000,0000,0000,,then the conclusion must be true.
Dialogue: 0,0:05:12.66,0:05:14.87,Default,,0000,0000,0000,,We could even use undefined terms.
Dialogue: 0,0:05:14.87,0:05:18.03,Default,,0000,0000,0000,,(P1): All sliff are splat.
Dialogue: 0,0:05:18.03,0:05:21.02,Default,,0000,0000,0000,,(P2): Sniff is a sliff.
Dialogue: 0,0:05:21.02,0:05:24.89,Default,,0000,0000,0000,,Conclusion: Therefore, sniff is a splat.
Dialogue: 0,0:05:24.89,0:05:28.63,Default,,0000,0000,0000,,Again, although the truth of\Nthe premises is undefined,
Dialogue: 0,0:05:28.63,0:05:30.34,Default,,0000,0000,0000,,we have a valid argument.
Dialogue: 0,0:05:30.34,0:05:32.97,Default,,0000,0000,0000,,This is just one type\Nof valid argument form,
Dialogue: 0,0:05:32.97,0:05:34.33,Default,,0000,0000,0000,,and you can learn about others
Dialogue: 0,0:05:34.33,0:05:35.96,Default,,0000,0000,0000,,in upcoming videos.
Dialogue: 0,0:05:35.96,0:05:37.48,Default,,0000,0000,0000,,Note now what it means
Dialogue: 0,0:05:37.48,0:05:39.32,Default,,0000,0000,0000,,for an argument to be invalid.
Dialogue: 0,0:05:39.32,0:05:41.66,Default,,0000,0000,0000,,The truth of the argument's premises
Dialogue: 0,0:05:41.66,0:05:44.72,Default,,0000,0000,0000,,does not entail the\Ntruth of the conclusion.
Dialogue: 0,0:05:45.52,0:05:46.60,Default,,0000,0000,0000,,For instance:
Dialogue: 0,0:05:46.60,0:05:49.60,Default,,0000,0000,0000,,(P1): All dogs have fur.
Dialogue: 0,0:05:49.60,0:05:52.48,Default,,0000,0000,0000,,(P2): Claire has a lot of fur.
Dialogue: 0,0:05:52.48,0:05:56.11,Default,,0000,0000,0000,,Conclusion: Therefore, Claire is a dog.
Dialogue: 0,0:05:56.11,0:05:57.75,Default,,0000,0000,0000,,Now, it could be the case
Dialogue: 0,0:05:57.75,0:05:59.80,Default,,0000,0000,0000,,that all of the premises in this argument
Dialogue: 0,0:05:59.80,0:06:03.14,Default,,0000,0000,0000,,are true, but the conclusion false.
Dialogue: 0,0:06:03.14,0:06:04.54,Default,,0000,0000,0000,,The truth of this conclusion,
Dialogue: 0,0:06:04.54,0:06:05.49,Default,,0000,0000,0000,,in other words,
Dialogue: 0,0:06:05.49,0:06:08.50,Default,,0000,0000,0000,,does not follow from the premises, right?
Dialogue: 0,0:06:08.50,0:06:11.91,Default,,0000,0000,0000,,Because cats also have a lot of fur.
Dialogue: 0,0:06:11.91,0:06:15.05,Default,,0000,0000,0000,,So this is an invalid argument.
Dialogue: 0,0:06:15.62,0:06:18.67,Default,,0000,0000,0000,,You may wonder why\Nvalidity matters at all,
Dialogue: 0,0:06:18.67,0:06:21.47,Default,,0000,0000,0000,,if the truth of the\Npremises doesn't matter.
Dialogue: 0,0:06:21.47,0:06:22.71,Default,,0000,0000,0000,,This is a good question to ask,
Dialogue: 0,0:06:22.71,0:06:24.72,Default,,0000,0000,0000,,and it deserves a long discussion.
Dialogue: 0,0:06:24.72,0:06:26.93,Default,,0000,0000,0000,,But the short answer is\Nthat validity is used
Dialogue: 0,0:06:26.93,0:06:29.87,Default,,0000,0000,0000,,to determine whether or\Nnot an argument obeys
Dialogue: 0,0:06:29.87,0:06:33.43,Default,,0000,0000,0000,,valid inference rules, the\Nlaws of deductive logic.
Dialogue: 0,0:06:33.43,0:06:36.75,Default,,0000,0000,0000,,That is, we are ensuring that\Ninferences in the argument
Dialogue: 0,0:06:36.75,0:06:39.06,Default,,0000,0000,0000,,are good inferences to make.
Dialogue: 0,0:06:39.75,0:06:41.63,Default,,0000,0000,0000,,I'll leave you with one last example,
Dialogue: 0,0:06:41.63,0:06:42.73,Default,,0000,0000,0000,,and ask you to determine
Dialogue: 0,0:06:42.73,0:06:45.06,Default,,0000,0000,0000,,its validity or invalidity.
Dialogue: 0,0:06:45.50,0:06:48.61,Default,,0000,0000,0000,,(P1): All fruit is a chair.
Dialogue: 0,0:06:48.61,0:06:51.84,Default,,0000,0000,0000,,(P2): Square is a chair.
Dialogue: 0,0:06:51.84,0:06:55.73,Default,,0000,0000,0000,,Conclusion: Therefore, square is a fruit.
Dialogue: 0,0:06:55.73,0:06:57.41,Default,,0000,0000,0000,,What do you think?