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Lecture 4

Lecture 4 Philosophy 275 Validity vs Invalidity.docx

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University of Calgary
PHIL 275
Yoshiki Kobasigawa

th Lecture 4 Validity vs Invalid September 16 , 2013 Valid arguments are deductive arguments. These don’t actually have to do with the actual truth of the premises. It’s a deductive argument where assuming the truth of the premises, the truth of the conclusion is guaranteed. Valid Argument Form: All F are G. All G are H. So All F are H. It implies that each is which each other. {H[G(f)]} There is another, similar form: All F are G. No G are H. So no F are H. In this case, the groups are separate: [G(F)] (H) Example: All humans are birds. No birds are tennis players. So no humans are tennis players. Invalid Argument Form: Arguments with a true premises and a true conclusion yet are invalid. In order for it to be a valid argument, you have to determine if it’s possible for the conclusion to be false. This makes for an invalid argument. This reasoning has the form: All F are G. All H are G. So all F are H. In this case, you have it visually: {G [F] [H]} or {G [F[]H]}. No F are necessarily H. (Think Venn diagram for the second possibility.) Example: All swans are animals. All birds are animals. So all swans are birds. In order for an argument to be valid, the truth of the premises must guarantee the truth of the conclusion. Counter example to an argument. Counter examples establish that an argument is invalid. 1) Identify the form of the argument. a. Replacing the terms in the argument with letters. (terms are underlined in the previous example) 2) Replace these letters with other terms where
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