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An Introduction to Critical Thinking and Creativity - always yours

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80 VALID AND SOUND ARGUMENTS<br />

9.2.5 Constructive dilemma<br />

A pattern of valid argument with three premises:<br />

PorQ.<br />

If P then R.<br />

If Q then S.<br />

ROTS.<br />

Either the president is lying, or he is telling the truth.<br />

If the president is lying, then he is wicked.<br />

If the president is telling the truth, then he is mad.<br />

Either the president is wicked, or he is mad.<br />

When R is the same as S, this is an equally valid pattern:<br />

PorQ.<br />

If P then R.<br />

If Q then R.<br />

R.<br />

Either our actions are r<strong>and</strong>om, or our actions are determined.<br />

If our actions are r<strong>and</strong>om, we do not have free will.<br />

If our actions are determined, we do not have free will.<br />

We do not have free will.<br />

9.2.6 Destructive dilemma<br />

See if you can come up with your own example of this pattern of valid argument:<br />

Not-Pornot-S.<br />

IfPthenP.<br />

If Q then S.<br />

Not-P or not-Q.<br />

9.2.7 Reductio ad absurdum<br />

Reductio ad absurdum is Latin for "reduced <strong>to</strong> absurdity." It is a method for showing<br />

that a certain statement S is false:

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