1. Chapter 10 Evaluating Premises: Self-Evidence, Consistency, Indirect Proof www.criticalthinking1ce.nelson.com Invitation to Critical Thinking First Canadian Edition Joel Rudinow Vincent E. Barry Mark Letteri

2. © 2008 by Nelson, a division of Thomson Canada Limited 10-2 Overview Evaluating Premises: Self-evidence, Consistency, Indirect Proof  Tautologies  Truisms by Definition  Contingent Claims  Values Relativism  Self-evident Claims  Beyond Self-evident Claims  Consistency  Implications

3. © 2008 by Nelson, a division of Thomson Canada Limited 10-3 Evaluating Premises  All arguments have to start somewhere  Every argument will have unsupported premises  When designing and constructing an argument, establish as firm a foundation as possible  Experienced arguers use the most “basic” premises as claims as uncontroversial as uncontroversial as easy to accept as easy to accept as hard to challenge or refute as hard to challenge or refute  Two “basic” premises or claims are tautologies and truisms by definition

4. © 2008 by Nelson, a division of Thomson Canada Limited 10-4 Tautologies  A claim that you can't deny without formally contradicting yourself considered to be necessarily true considered to be necessarily true though tautologies always carry the value “true,” they don't convey much information though tautologies always carry the value “true,” they don't convey much information

5. © 2008 by Nelson, a division of Thomson Canada Limited 10-5 Example of Tautology  Either Conrad Black knew about major financial irregularities in his company or he did not.  If he did know about such irregularities, then he is guilty of criminal conduct and therefore unworthy of his position.  If he did not know about major financial irregularities, then he is not in control of his own company and is therefore unworthy of his position.  Therefore, either way, he is definitely unworthy of his position.

6. © 2008 by Nelson, a division of Thomson Canada Limited 10-6 Truisms by Definition  The terms of the claim reveal the truth of the statement  Anyone who understands the meanings of the terms in the claim will immediately recognize any such statement as true

7. © 2008 by Nelson, a division of Thomson Canada Limited 10-7 Examples of Truism by Definition  Murder is a form of homicide  All bachelors are unmarried

8. © 2008 by Nelson, a division of Thomson Canada Limited 10-8 Contingent Claims  Claims that are neither self-contradictory nor necessarily true are called contingent.  The truth or falsity of contingent claims depends on something outside themselves. Something beyond their formal structures Something beyond their formal structures Something beyond the meanings of their terms Something beyond the meanings of their terms

9. © 2008 by Nelson, a division of Thomson Canada Limited 10-9 When Evaluating Contingent Claims, ask:  What kind of claim is being made here?  Does the premise make a factual claim or an evaluative claim?  Does it offer an interpretation?  What sort of issue does it raise?  What sorts of additional support might this claim need?

10. © 2008 by Nelson, a division of Thomson Canada Limited 10-10 Examples of Contingent Claims  All persons are free and equal  Not all persons are free and equal  Some persons are free and equal

11. © 2008 by Nelson, a division of Thomson Canada Limited 10-11 Values Relativism  More reasonable and understandable than relativism in general because evaluative issues (which cannot be resolved by doing science or looking things up generally) are harder to resolve than factual issues. because evaluative issues (which cannot be resolved by doing science or looking things up generally) are harder to resolve than factual issues.  Some hold that values relativism is impossible due to the essential difference and an unbridgeable gap between facts and values.  The fact that evaluative claims can't be established empirically doesn’t prove that they can't be established at all.  To resolve evaluative claims, consider and evaluate the best available arguments on all sides of the issue.

12. © 2008 by Nelson, a division of Thomson Canada Limited 10-12 Self-evident Claims  Self-evident claims: the supporting claims are no more basic or evident than what they support  To support them by appeal to further observations would be no more basic or evident than what they support  Unless good reason exists for doubting them, such claims may be taken as self- evident

13. © 2008 by Nelson, a division of Thomson Canada Limited 10-13 Examples of Self-evident Claims  Human life is precious  Peace is precious  Freedom is better than colonial bondage as a way of life  Basic human rights belong equally to each and every human being

14. © 2008 by Nelson, a division of Thomson Canada Limited 10-14 Beyond Self-evident Claims  To determine whether a claim is beyond self-evident, ask: Is it an interpretation? Is it an interpretation? This means other interpretations are possible. This means other interpretations are possible. This means that the issue is interpretive. This means that the issue is interpretive.  An interpretation is the kind of claim one can always legitimately be challenged to support argumentatively: No single simple procedure exists for resolving interpretive issues. No single simple procedure exists for resolving interpretive issues. No single simple procedure exists for establishing interpretive claims as premises in an argument. No single simple procedure exists for establishing interpretive claims as premises in an argument. To evaluate it: To evaluate it: Consider the best arguments that can be made for and against the claim.Consider the best arguments that can be made for and against the claim. Weigh up the arguments and the evidence on all sides.Weigh up the arguments and the evidence on all sides.

15. © 2008 by Nelson, a division of Thomson Canada Limited 10-15 Consistency of Claims  Consistency is crucial to our understanding of deductive validity, but inconsistency is always a sign that something is wrong somewhere.  If a given set of claims as a group is internally inconsistent, then although you may not know which of the premises is false, you know they can't all be true.

16. © 2008 by Nelson, a division of Thomson Canada Limited 10-16 Implications of Claims  If a claim leads by implication to any further claim that is self-contradictory, or otherwise absurd or known to be false, then good reason exists to doubt the claim.  This strategy has traditionally been known by its Latin name reductio ad absurdum (which means to reduce to absurdity).  The strategy can also be inverted to use in defence of a position.