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Unit 11 Reasoning The Validity of Arguments

Unit 11 Reasoning The Validity of Arguments. Arguments. Assertions : unsupported declaration of a belief Prejudice : a view without evidence for or against Premises: explicit evidence that lead to a conclusion Conclusion : the logical outcome of a set of premises . Argument structure.

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Unit 11 Reasoning The Validity of Arguments

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  1. Unit 11 ReasoningThe Validity of Arguments

  2. Arguments • Assertions: unsupported declaration of a belief • Prejudice: a view without evidence for or against • Premises: explicit evidence that lead to a conclusion • Conclusion: the logical outcome of a set of premises

  3. Argument structure • premises • A Conclusion • All humans are mortal. • Ali is human • Ali is mortal • A case can be (in)valid, (un)sound.

  4. Validity and Soundness • Argument #1 : Barbie is over 90 years old. So Barbie is over 20 years old. • Argument #2 : Barbie is over 20 years old. So Barbie is over 90 years old. • intuitively, the conclusion of the first argument follows from the premise, whereas the conclusion of the second argument does not follow from its premise. But how should we explain the difference between the two arguments more precisely? Here is a thought : In the first argument, if the premise is indeed true, then the conclusion cannot be false. On the other hand, even if the premise in the second argument is true, there is no guarantee that the conclusion must also be true. For example, Barbie could be 30 years old.

  5. An argument is valid if and only if there is no logically possible situation where all the premises are true and the conclusion is false at the same time. • Argument #1 Valid • Argument #2 invalid

  6. Validity and truth • All pigs can fly. Anything that can fly can swim. So all pigs can swim. • Although the two premises of this argument are false, this is actually a valid argument. To evaluate its validity, ask yourself whether it is possible to come up with a situation where all the premises are true and the conclusion is false. Of course, the answer is 'no'. If pigs can indeed fly, and if anything that can fly can also swim, then it must be the case that all pigs can swim. • The premises and the conclusion of a valid argument can all be false.

  7. Validity and truth • validity is not about the actual truth or falsity of the premises or the conclusion. Validity is about the logical connectionbetween the premises and the conclusion. • A valid argument is one where the truth of the premises guarantees the truth of the conclusion,but validity does not guarantee that the premises are in fact true. • All that validity tells us is that if the premises are true, the conclusion must also be true.

  8. Showing that an argument is invalid • Adam loves Beth. Beth loves Cathy. So Adam loves Cathy. • This argument is not valid, for it is possible that the premises are true and yet the conclusion is false. Perhaps Adam loves Beth but does not want Beth to love anyone else. So Adam actually hates Cathy. The mere possibility of such a situation is enough to show that the argument is not valid. • An argument can be invalid even if the conclusion and the premises are all actually true.

  9. True= statement, valid = argument

  10. Practice • http://philosophy.hku.hk/think/arg/valid1.php

  11. Soundness • So even if we are given a valid argument, we still need to be careful before accepting the conclusion, since a valid argument might contain a false conclusion. What we need to check further is of course whether the premises are true. If an argument is valid, and all the premises are true, then it is called a sound argument

  12. So given that a sound argument is valid and has true premises, its conclusion must also be true. • So if you have determined that an argument is indeed sound, you can certainly accept the conclusion. • An argument that is not sound is an unsound argument. If an argument is unsound, it might be that it is invalid, or maybe it has at least one false premise, or both.

  13. Possibilities • Valid and sound • Valid and unsound • Invalid and sound • Invalid and unsound • Practice • http://philosophy.hku.hk/think/arg/valid-ex3.php

  14. fallacy • An argument can be a fallacy: it seems sound but it is biased • All witches keep black cats • My neighbor keeps a black cat • So my neighbor is a witch • Formal fallacy are unsound arguments.

  15. Thought experiements • Using imaginary cases to support an argument • Bentham wrote an article on “the status of animals” in which he refutes the argument for granting rights only to those who can speak or reason. Rather he places his argument on those who can suffer.

  16. Reduction ad absurdum • “A reduction to absurdity” is a technique used in thinking to show that something is wrong. • Babies have rights • Rights are based on the ability to speak or reason • Babies can do neither • So babies do not have rights. • This absurd because it has contradiction. • He showed the fallacy of having rights based on speaking or reasoning.

  17. Logic: arguments • premises • conclusion • validity • soundness • fallacy • thought experiments • reduction to absurdity

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