lecture2 - Philosophy 3 Critical Thinking Course website...

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    Philosophy 3: Critical Thinking Course website: http://www.philosophy.ucsb.edu/websites/phil3/ Homework exercises, due Monday: Layman, ch. 1.1 A 2, 5, 10, 13, 15, 17, 20 B 2, 5, 6, 9, 12, 14, 15, 20, 24, 26, 29 C 1, 3, 5, 7, 11, 14, 15 D 3, 4, 6, 9, 14 1.4 B 1, 3, 5, 8, 10, 12, 14 C 2, 5, 6, 8, 11, 15, 17 D 1, 2, 5, 6, 9
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    Recap This class is about methods for distinguishing good arguments from bad arguments An argument is a series of statements one of which (the conclusion ) is claimed to be supported by the others (the premises ) An argument is good when it is truth-preserving. An argument is truth-preserving when it won’t lead us from true premises to a false conclusion – if the premises are true, the conclusion will be too.
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    Introducing Validity We left off talking about this argument: 1) To see the movie, I need to leave 30mins before the movie starts. 2) The movie starts at 7pm. So, 3) To see the movie, I need to leave at 6.30pm. We said it was a good argument because if the premises are true, the conclusion will be too.
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    Introducing Validity In fact, something more is true. If the premises of this argument are true, the conclusion must be true. As we can also put it, the conclusion follows from the premises; the premises entail the conclusion. Arguments like this are valid .
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    More Examples 1) All men are mortal 2) Socrates is a man So, 3) Socrates is mortal 1) Hillary will win the nomination only if Obama loses. 2) Obama will not lose So, 3) Hillary will not win the nomination In both examples, if the premises are true, the conclusion must be true too.
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    The intuitive idea: an argument is valid when the conclusion follows from the premises. Note: ‘valid’ is used as a technical term. It can be applied to arguments but not to statements. By contrast, ‘true’ and ‘false’ can be applied to statements but not to arguments.
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    A precise definition of validity An argument is valid if and only if it’s necessary that if the premises are true, then the conclusion is true.
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