Lecture 4_Validity, soundness, strength, and cogency

Lecture 4_Validity, soundness, strength, and cogency -...

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Unformatted text preview: Lecture 4 Validity, Soundness, Strength, and Cogency Patrick Maher Philosophy 102 Spring 2009 Validity Definition A valid argument is an argument in which the conclusion follows necessarily from the premises (i.e., it is impossible for the premises to be true and the conclusion false). Arguments that are not valid are invalid . Deductive arguments are claimed to be valid. Examples All deliberate killing of helpless persons is wrong. Euthanasia is a deliberate killing of a helpless person. Therefore, euthanasia is wrong. Valid If this is sulfur then it will burn with a yellow flame. It burns with a yellow flame. Therefore, it is sulfur. Invalid Validity versus truth In logic, ‘valid’ and ‘true’ have different meanings and must not be confused. They apply to different things Arguments are valid or invalid; they are not true or false. Statements are true or false; they are not valid or invalid. Whether an argument is valid usually cannot be determined from the truth or falsity of its premises or conclusion. Premises Conclusion Validity T T ? T F Invalid F T ?...
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Lecture 4_Validity, soundness, strength, and cogency -...

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