1. Repairing Arguments

2. Need to repair arguments  We can and must rewrite many arguments by adding an unstated premise or even an unstated conclusion.

3. The Principle of Rational Discussion  If you recognise that an argument is good, then it is irrational not to accept the conclusion.

4. The Principle of Rational Discussion  We assume that the other person who is discussing with us or whose arguments we are reading:  Knows about the subject under discussion  Is able and willing to reason well.  Is not lying

5. Example  When you leave your car for repairs, you agree to the mechanic’s suggestion for repairs.  You assume that the mechanic has specialised knowledge  You assume that he is not cheating

6. The Principle of Rational Discussion  The Principle of Rational Discussion does not instruct us to give other people the benefit of the doubt.  It summarizes the necessary conditions for us to be reasoning with someone

7. Why apply the principles of rational discussion? If you don’t:  You are denying the essentials of democracy  You are likely to undermine your ability to evaluate arguments  You are not as likely to convince others

8. Why apply the principles of rational discussion?  A representative democracy is built on the idea that the populace as a whole can choose good men and women to write laws by which they can agree to live.

9. Why apply the principles of rational discussion?  It is only by constantly striving to base our political discussions on good arguments that we have any hope of living in a just and efficient society

10. Abraham Lincoln  If you once forfeit the confidence of your fellow citizens, you can never regain their respect and esteem. It is true that you may fool all the people some of the time; you can even fool some of the people all the time; but you can’t fool all of the people all the time.

11. Guide to repairing arguments  If an argument is defective, we are justified in adding a premise or conclusion if it satisfies all three of the following:  The argument becomes stronger or valid.  The premise is plausible and would seem plausible to other person.  The premise is more plausible than the conclusion.

12. Example  I was wondering what kind of pet Dick has. It must be a dog.  How do you know?  Because I heard it barking last night. (Premise to be added: “All pets that bark are dogs.”) – this is false “Almost all pets that bark are dogs.”

13. Guide to repairing arguments  When you find a possible way for the premises to be true and the conclusion false, you try to eliminate it by adding a premise – of course a plausible one.  As you eliminate ways in which the premises could be true and the conclusion false, you make the argument better.

14. Indicator word  An indicator word is a word or phrase added to a claim to tell us the role of the claim in an argument or what the speaker thinks of the claim or argument.

15. Indicator word  Conclusion indicators  Therefore  Hence  So  Thus  Consequently  We can then derive  It follows that  Premise indicators  Since  Because  For  In as much as  Given that  Suppose that  It follows from

16. Example 1  No dogs meows. So Spot does not meow.  Analysis: “Spot is a dog” is the only premise that will make this a valid or strong argument. So we add that. Then, since this new claim is true, the argument is good.

17. Example 2  Suzy: All professors teach. So Ms. Han is a professor. Analysis: The obvious claim to add is “Ms Han teaches.” Then we get: All professors teach. Ms Han teaches. So Ms Han is a professor. (This is weak as Ms Han can be an instructor, tutor or administrator)

18. Unrepairable Arguments  There is no argument there.  The argument is so lacking in coherence that there’s nothing obvious to add.  A premise it uses is false or very dubious, or some of the premises are contradictory and cannot be deleted.

19. Unrepairable Arguments  The obvious premise to add would make the argument weak.  The obvious premise to add to make the argument strong or valid is false.  The conclusion is clearly false.

20. Relevance (bad arguments)  Environmentalists should not be allowed to tell us what to do.  The government should not be allowed to tell us what to do.  Therefore, we should go ahead and allow logging in old-growth forests. (Bad argument because his premises are irrelevant to the conclusion)

21. Irrelevant premise  A premise is irrelevant if you can delete it and the argument isn’t any weaker.

22. THANK YOU