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Dialogue: 0,0:00:02.66,0:00:08.10,Default,,0000,0000,0000,,The next question has probably been\Nbothering you ever since very early on in
Dialogue: 0,0:00:08.10,0:00:13.87,Default,,0000,0000,0000,,the previous lecture. Namely, if valid\Narguments can have false premises, then
Dialogue: 0,0:00:13.87,0:00:19.15,Default,,0000,0000,0000,,what good are they? Sure, there's this\Ntechnical logician's notion of a valid
Dialogue: 0,0:00:19.15,0:00:24.92,Default,,0000,0000,0000,,argument, but why should we care whether\Narguments are valid if valid arguments can
Dialogue: 0,0:00:24.92,0:00:30.82,Default,,0000,0000,0000,,be really bad? Validity might be necessary\Nfor an argument to be good or at least for
Dialogue: 0,0:00:30.82,0:00:35.96,Default,,0000,0000,0000,,a deductive argument to be good because\Nremember, there are also inductive
Dialogue: 0,0:00:35.96,0:00:42.41,Default,,0000,0000,0000,,arguments. But even though it's necessary,\Nit's not enough. You can have a horrible
Dialogue: 0,0:00:42.41,0:00:48.26,Default,,0000,0000,0000,,argument but still valid. Well, the great\Nthing about validity is that when you add
Dialogue: 0,0:00:48.26,0:00:54.04,Default,,0000,0000,0000,,true premises to a valid argument, then\Nyou get something that really is valuable,
Dialogue: 0,0:00:54.04,0:00:58.97,Default,,0000,0000,0000,,which we're going to call a sound\Nargument. Because if you know that the
Dialogue: 0,0:00:58.97,0:01:04.82,Default,,0000,0000,0000,,premises are true and you also know that\Nit's not possible for the premises to be
Dialogue: 0,0:01:04.82,0:01:10.79,Default,,0000,0000,0000,,true and the conclusion, false, then, you\Nknow, the conclusion must be true. So, in
Dialogue: 0,0:01:10.79,0:01:16.95,Default,,0000,0000,0000,,a sound argument, the conclusion has to be\Ntrue. And that is what makes it valuable
Dialogue: 0,0:01:16.95,0:01:23.19,Default,,0000,0000,0000,,cuz if we can get a deductive argument to\Nbe sound, then you really got something.
Dialogue: 0,0:01:23.19,0:01:28.98,Default,,0000,0000,0000,,What you've got is a true conclusion.\NOfficially then, a sound argument is one
Dialogue: 0,0:01:28.98,0:01:35.17,Default,,0000,0000,0000,,where the premises are true and the\Nargument is valid. And we've got the same
Dialogue: 0,0:01:35.17,0:01:40.57,Default,,0000,0000,0000,,combinations of truth and falsity as\Npossibilities that we had in valid
Dialogue: 0,0:01:40.57,0:01:46.43,Default,,0000,0000,0000,,arguments. You can have both premises and\Nconclusion are true and then, if it's
Dialogue: 0,0:01:46.43,0:01:52.29,Default,,0000,0000,0000,,valid, the argument is sound and if it's\Nnot valid, it's not. Or you can have the
Dialogue: 0,0:01:52.29,0:01:58.15,Default,,0000,0000,0000,,premises are true and the conclusions\Nfalse and then, it can't be valid. But if
Dialogue: 0,0:01:58.15,0:02:04.46,Default,,0000,0000,0000,,it's invalid, it's not sound. We can have\Nthe premises are false and the conclusions
Dialogue: 0,0:02:04.69,0:02:10.78,Default,,0000,0000,0000,,true. And then if it's valid, it's not\Nsound and if it's invalid, it's not sound.
Dialogue: 0,0:02:10.78,0:02:17.01,Default,,0000,0000,0000,,Or we can have both the premises and the\Nconclusion are false, and then, it's not
Dialogue: 0,0:02:17.01,0:02:22.78,Default,,0000,0000,0000,,going to be sound whether it's valid or\Nnot. So, the only combination, where it
Dialogue: 0,0:02:22.78,0:02:28.94,Default,,0000,0000,0000,,sound is when the premises are true and\Nthe argument is valid and, in that case,
Dialogue: 0,0:02:28.94,0:02:34.52,Default,,0000,0000,0000,,you know that the conclusion is true. What\Nabout lack of soundness? Well, there are
Dialogue: 0,0:02:34.52,0:02:39.75,Default,,0000,0000,0000,,two ways an argument yjay fail to be\Nsound, namely, either the argument can be
Dialogue: 0,0:02:39.75,0:02:44.78,Default,,0000,0000,0000,,invalid or one of its premises can be\Nfalse. So, it's a lot easier for an
Dialogue: 0,0:02:44.78,0:02:50.29,Default,,0000,0000,0000,,argument to be unsound. And we know that a\Ndeductive argument tries to be valid and,
Dialogue: 0,0:02:50.29,0:02:55.59,Default,,0000,0000,0000,,of course, it wants its premises to be\Ntrue so a deductive argument is trying to
Dialogue: 0,0:02:55.59,0:03:00.58,Default,,0000,0000,0000,,be sound. And when it fails to be sound,\Nit's not going to be any good. And the
Dialogue: 0,0:03:00.58,0:03:06.44,Default,,0000,0000,0000,,next question is how can you know? If you\Ndon't know whether the premises are true,
Dialogue: 0,0:03:06.44,0:03:11.06,Default,,0000,0000,0000,,you're not going to know whether the\Narguments sound. Well, not quite, because
Dialogue: 0,0:03:11.06,0:03:15.74,Default,,0000,0000,0000,,if you, if the argument's valid and you\Nknow it's valid, then you don't know
Dialogue: 0,0:03:15.74,0:03:20.55,Default,,0000,0000,0000,,whether it's sound unless you know the\Npremises are true. But if you that the
Dialogue: 0,0:03:20.55,0:03:25.24,Default,,0000,0000,0000,,argument is invalid, you already know it's\Nunsound, even if you don't know whether
Dialogue: 0,0:03:25.24,0:03:30.05,Default,,0000,0000,0000,,the premises are true. So, if you think\Nabout it, that shows why you want to be
Dialogue: 0,0:03:30.05,0:03:34.87,Default,,0000,0000,0000,,able to test for validity. Because if you\Ncan show the argument's invalid, then
Dialogue: 0,0:03:34.87,0:03:39.67,Default,,0000,0000,0000,,you're going to be able to, well, I know\Nit's unsound, regardless of what you think
Dialogue: 0,0:03:39.67,0:03:44.41,Default,,0000,0000,0000,,about whether the premises are true or\Nnot. So, there's going to be some value to
Dialogue: 0,0:03:44.41,0:03:49.08,Default,,0000,0000,0000,,validity, namely, if you can show it's\Ninvalid, you're going to show it's unsound
Dialogue: 0,0:03:49.08,0:03:54.50,Default,,0000,0000,0000,,and that means that the deductive argument\Ndidn't get what it wanted. So, validity is
Dialogue: 0,0:03:54.50,0:04:00.54,Default,,0000,0000,0000,,going to be necessary for soundness and\Nsoundness is going to be important because
Dialogue: 0,0:04:00.54,0:04:06.88,Default,,0000,0000,0000,,it guarantees the truth of the conclusion,\Nand then, validity derives its value from
Dialogue: 0,0:04:06.88,0:04:12.81,Default,,0000,0000,0000,,the fact that if it's not valid, it's not\Nsound. Okay. Now, there's a more to say
Dialogue: 0,0:04:12.81,0:04:17.23,Default,,0000,0000,0000,,about validity. And we'll say a lot more\Nabout validity when we get to a formal
Dialogue: 0,0:04:17.67,0:04:22.59,Default,,0000,0000,0000,,logic in the second part of this course.\NBut for now, we're just going to stick
Dialogue: 0,0:04:22.59,0:04:27.25,Default,,0000,0000,0000,,with this pretty intuitive notion of\Nvalidity and see how we can use this
Dialogue: 0,0:04:27.25,0:04:30.03,Default,,0000,0000,0000,,notion of validity to reconstruct\Narguments.