Three Acts of the Mind

Transcription

1 Three Acts of the Mind Mental Act: Verbal Expression: Simple Apprehension Judgment Deductive Inference Term Proposition Syllogism Slide 13-1 The Three Categories of Rules of Validity Slide 13-2 Terminological Quantitative Qualitative

2 Qualitative Rules These two rules are quantitative because they have to do with the quality of the statements in the syllogism. Slide 13-3 Qualitative Rules of Validity V. No conclusion can follow from two negative premises. VI. If the two premises are affirmative, the conclusion must also be affirmative. VII. If either premise is negative, the conclusion must be negative. Slide 13-4

3 Rule V: No conclusion can follow from two negative premises Slide 13-5 This rule prevents us, once again, from trying to say more in the conclusion than is justified in the premises. Application of Rule V No plants Pd are animals Md Some minerals Su are not animals Md Therefore, some minerals Su are not plants Pd Slide 13-6

4 No plants Pd are animals Md Some minerals Su are not animals Md Therefore, some minerals Su are not plants Pd Possibility #1 True premises True conclusion Plants Minerals Animals Possibility #2 True premises False conclusion Plants Minerals Animals Slide 13-7 The Fallacy of Exclusive Premises This fallacy is committed when a conclusion is erroneously derived from two negative premises. Slide 13-8

5 Fixing the Syllogism: No plants Pd are animals Md All animals Md have lungs Sd Therefore, some creatures that have lungs Su are not plants Pd Slide 13-9 Qualitative Rules of Validity V. No conclusion can follow from two negative premises. VI. If the two premises are affirmative, the conclusion must also be affirmative. VII. If either premise is negative, the conclusion must be negative. Slide 13-10

6 Rule VI: If the two premises are affirmative, the conclusion must also be affirmative Slide Application of Rule VI All men Pd are mortal Mu All mortals Md are subject to death Su Therefore, some beings that are subject to death Su are not men Pd Slide 13-12

7 All men Pd are mortal Mu All mortals Md make mistakes Su Therefore, some things that make mistakes Su are not Possibility #1 True premises True conclusion men Pd. Things that make mistakes Mortals Men Slide Possibility #2 True premises False conclusion Men Things that make mistakes Mortals The Fallacy of Drawing a Negative Conclusion from Affirmative Premises Slide This fallacy is committed when a negative conclusion is erroneously derived from two affirmative premises.

8 Fixing the Syllogism: All men Pd are mortal Mu All mortals Md are subject to death Su Therefore, some things that are subject to death Su are men Pu Slide Qualitative Rules of Validity V. No conclusion can follow from two negative premises. VI. If the two premises are affirmative, the conclusion must also be affirmative. VII. If either premise is negative, the conclusion must be negative. Slide 13-16

9 Rule VII: If either premise is negative, the conclusion must also be negative Slide Application of Rule VII All cannibals Pd are bloodthirsty Mu Some accountants Su are not bloodthirsty Md Therefore, some accountants Su are cannibals Pd Slide 13-18

10 All cannibals Pd are bloodthirsty Mu All accountants Sd are not bloodthirsty Md Therefore, some accountants Su are cannibals Pu Possibility #1 True premises False conclusion Bloodthirsty People Cannibals Accountants Slide The Fallacy of Drawing an Affirmative Conclusion from Negative Premises Slide This fallacy is committed when an affirmative conclusion is erroneously derived from two negative premises

11 Fixing the Syllogism All cannibals Pd are bloodthirsty Mu Some accountants Su are not bloodthirsty Md Therefore, some accountants Su are not cannibals Pd Slide Qualitative Rules of Validity V. No conclusion can follow from two negative premises. Slide VI. If the two premises are affirmative, the conclusion must also be affirmative. VII. If either premise is negative, the conclusion must be negative.