Fundamentals: Truth and validity발음듣기
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Fundamentals: Truth and validity
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(intro music) Hi! My name is Julianne Chung, and I am a graduate student at Yale University.발음듣기
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In this video, however, we are going to discuss just two good qualities that arguments can have that are particularly important for determining whether we should accept their conclusions.발음듣기
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The first is this: the premises of an argument may be true, that is, they may be in agreement with the facts.발음듣기
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In philosophy, truth and falsity are held to be properties of statements, but not arguments.발음듣기
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In other words, an argument is valid just in case the truth of its premises guarantees the truth of its conclusion.발음듣기
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In philosophy, validity and invalidity are held to be properties of arguments, but not statements.발음듣기
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To see the difference between these properties, it will be helpful to look at some examples, all of which involve my good friend Julia's dog, Split.발음듣기
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In this argument, not only are the premises true, but the conclusion follows logically from them.발음듣기
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Here, the premises are obviously true, but the conclusion does not follow logically from them.발음듣기
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Of course, this argument is clearly unacceptable, because its conclusion is obviously false.발음듣기
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However, sometimes arguments can have true premises, as well as true conclusions, but still be invalid because the conclusions do not follow logically from them.발음듣기
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Because of this, it is important that we are careful to ensure that the conclusion really does follow from the premises under consideration when we are evaluating an argument.발음듣기
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Here, the first premise is false, but the reasoning is valid, because the conclusion follows logically from the premises.발음듣기
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Notice, too, that just as in the last example, the conclusion of this argument may happen to be true, although the argument does not establish that it is. Alright, just one more example.발음듣기
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In this example, not only is premise two false, but the conclusion does not follow logically from the premises.발음듣기
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You've probably already noticed that truth and falsity, as well as validity and invalidity, can appear in various combinations in an argument, giving rise to four possibilities.발음듣기
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Possibility one: we may have our facts right, our premises are true, and we may use them properly. Our reasoning is valid.발음듣기
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Possibility two: we may have our facts right, our premises are true, and we may use them improperly. Our reasoning is invalid.발음듣기
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Possibility three: we may have our facts wrong, some of our premises are false, and we may use them properly. Our reasoning is valid.발음듣기
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And finally, possibility four: we may have our facts wrong, some of our premises are false, and we may use them improperly. Our reasoning is invalid.발음듣기
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When we are evaluating an argument, we should only accept its conclusions if the first possibility obtains.발음듣기
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Because of this, you might be wondering why we should be at all interested in arguments that are valid, but whose premises are false?발음듣기
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But being able to validly infer the conclusions that would follow from such premises if they were true sometimes enables us to judge whether they are true.발음듣기
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This is because validly inferring a conclusion that we know to be false from a given set of premises will tell us that one of our premises must be false too.발음듣기
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After all, a false conclusion cannot validly be deduced from true premises. Consider the following example.발음듣기
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Say that John calls his boss at work one day, and tells her that he is in bed with a terrible case of the flu.발음듣기
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She could conclude that John is not in bed with a terrible case of the flu. It seems he lied.발음듣기
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That said, hopefully it suffices to show that we often use reasoning like this to figure out whether claims are true or false.발음듣기
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Thus, it is indeed often very useful for us to know whether an argument is valid, even if we don't know whether its premises are true.발음듣기
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