Deductive vs Non-Deductive Arguments 2018.docx

Deductive vs Non-Deductive Arguments 2018.docx - Deductive...

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Deductive vs Non-Deductive Arguments page 1 As we’ve already discussed, in order for an argument to be good, one of the conditions (Condition 3) that it must meet is that the premise(s) have to be properly related to the conclusion . When you are asking whether this condition has been met, it is helpful to decide what sort of argument is intended. Sometimes people intend to give what are called “ deductive arguments and sometimes people intend to give what are called “ non-deductive ” arguments. If the argument is deductive, then the standard that you use to judge whether the premises are properly related to the conclusion is different from the standard you use for a non-deductive argument. Watch this video on the difference between deductive arguments and non-deductive arguments . The narrator of the video, Geoff Pynn, uses the term ‘ampliative’ instead of ‘non- deductive’. Assume these terms mean the same thing. He also starts talking about the difference at approximately minute 5:52 in the video. I would recommend watching the entire video, because the beginning of it is a good review. Recall that Condition 1 of our three conditions for a “good” argument was that the premises have to be true , or likely to be true . But whether or not an argument passes Condition 1, it can pass Condition 3 by having premises that are “properly related” to the conclusion. What does this mean? For non-deductive arguments (often called “inductive”), it means that the premises, if granted as true, make the conclusion likely . Note the word “likely” here. It means probable, rather than certain. A non-deductive argument sets out to make its conclusion seem likely, if one supposes its premises to be true. Here is an example of a non-deductive argument. (We will return to non-deductive arguments later in this lesson.) 83% of Americans are non-smokers. Lester is an American. Therefore Lester probably is a non-smoker. 1 If you suppose that (or imagine a world in which) 83% of Americans are non-smokers and Lester is an American, then you will conclude it is likely that Lester is a non-smoker. But it is still possible that he isn’t. The odds say he doesn’t smoke, but he could be one of the 17% who do smoke. DEDUCTIVE ARGUMENTS A deductive argument goes farther. “Likely” is not good enough for a deductive argument. It wants to guarantee the truth of its conclusion, not just render it likely. A deductive argument, if it could talk, would say: “If you assert my premises to be true, then you will find that what you have already asserted necessarily (not just probably) makes my conclusion true.” 1 Some students have astutely asked: if the word ‘probably’ is included in the conclusion, why doesn’t that make the argument deductive , because the (weakened) conclusion is now guaranteed by the premises? This raises a difficult philosophical issue about non-deductive arguments, to which we will give a brief answer at the close of this lesson. (See footnote 6 on page 7.)
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