Methods of Legal Argument

1. Methods of Legal Argument A course in Logic Prof. Dr. John JA Burke

2. Introduction to Course Required Text Copi & Cohen, Introduction to Logic, Pearson/Prentice Hall, 13th ed. Materials http://www.eilfe.com Office 3rd Floor Library Office Tel 2577 Hours By appointment

3. Subjects of Study Argument � Basic Concepts Informal Logic � Fallacies Deduction Induction Analogical Reasoning Causal Reasoning Science and Probability

4. Methodology Read Text Complete exercises after text material Diagram and analyse arguments Read cases and evaluate persuasiveness of reasoning Student partipcation

5. Switch What is logic

6. Logic The study of the methods and principles to distinguish correct from incorrect reasoning Main Question Does the conclusion reached follow from the premises assumed Logic is used to support assertions

7. Propositions and Arguments Propositions assert that something is the case Every proposition is either true or false Though its truth value may be unknown Example There is life on some other planet in our galaxy Is this a proposition

8. Not Propositions Questions Commands Exclamations Why Because they are neither true nor false The term �statement� is used as synonym for �proposition�

9. Types of Propositions Simple Compound A proposition containing other propositions Example The Amazon Basin produces roughly 20% of the Earth�s oxygen, creates much of its own rainfall, and harbours many unknown species Disjunctive Either/or statements No single truth is asserted On or the other must be true Hypothetical/Conditional If, then statements Example: �If God did not exist, it would be necessary to invent him�

10. Arguments Constructed from propositions Propositions are building blocks An argument affirms one proposition on the basis of other propositions This activity requires an inference An inference is a process tying together propositions Logicians analyse clusters of propositions to see if they constitute a correct argument

11. What is an argument An argument is a cluster of propositions related to one another in such a way that one proposition � the Conclusion � follows from the other propositions � the Premises There are many kinds of arguments Politics Ethics Science Law Logicians are concerned about: Form Quality

12. Simplest One Premise/One Conclusion Example �No one was present when life first appeared on earth. Therefore any statement about life�s origins should be considered as theory, not fact�. What is Premise; What is Conclusion What would a scientist have to say about this proposition?

13. What is Premise, Conclusion �Every law is an evil, for every law is an infraction of liberty�. Jeremy Bentham �I do not want people to be very agreeable, that would save me the trouble of liking them�. Jane Austen �If I did not want people to be agreeable, that would save me the trouble of liking them�.

14. Switch Validity and Truth

15. Deductive Arguments We must distinguish between Validity/Invalidity of an Argument Truth/Falsity of Propositions Deductive argument One single conclusion Set of premises Deductive argument is valid if Conclusion follows with logical necessity from premises [Relation is the Key]

16. Concept of Truth Does not apply to arguments Propositions within the argument may be true/false But the argument is neither true nor false Heart of deductive logic Study of relations between true/false propositions and valid/invalid arguments An argument may be valid even if one of its propositions is false or is in dispute

17. Dred Scott Decision Supreme Court in 19th Century rules that slaves who had escaped to the North must be returned to their owners in the South The Syllogism Nothing in the Constitution or laws of any State can destroy a right distinctly and expressly affirmed in the Constitution of the United States The right od property in a slave is distinctly and expressly affirmed in the constitution of the United States Therefore, nothing in the Constitution or laws of any State can destroy the property in a slave Abraham Lincoln attacked the reasoning of the Supreme Court How? Has the conclusion been established? Is the argument valid or invalid

18. Griswold v. Connecticut Read the case What are the premises What is the conclusion Is the argument valid/invalid Are the propositions true/false

19. Illustrations of Arguments I All mammals have lungs All whales are mammals Therefore all whales have lungs Question Are premises true? Is conclusion true Is argument valid?

20. Illustration II All 4-legged creatures have wings All spiders have 4 legs Therefore all spiders have wings Question Are premises true? Is conclusion true Is argument valid?

21. Illustration III If I owned all Gold in Fort Knox, I would be wealthy I do not own all Gold in Fort Knox Therefore I am not wealthy Question Are premises true? Is conclusion true Is argument valid?

22. Illustration IV If Bill Gates owned all Gold in Fort Knox, then Bill Gates would be wealthy Bill Gates does not own all Gold in Fort Knox Therefore Bill Gates is not wealthy Question Are premises true? Is conclusion true Is argument valid?

23. Illustration V All fish are mammals All whales are fish Therefore all whales are mammals Question Are premises true? Is conclusion true Is argument valid?

24. Illustration VI All mammals have wings All whales have wings Therefore all whales are mammals Question Are premises true? Is conclusion true Is argument valid?

25. Illustration VII All mammals have wings All whales have wings Therefore all mammals are whales Question Are premises true? Is conclusion true Is argument valid?

26. Invalid Arguments