Logic ~ evaluating arguments

1. Logic ~ evaluating arguments Chapter 2 - Evaluating Deductive Arguments • What we are doing in this section? • evaluating arguments • Cf. analyzing arguments • Evaluation - general considerations • 1. Are two types of evaluation which applies to all arguments Evaluating deductive arguments - 1

2. Logic ~ evaluating arguments • (1) Whether the statements in an argument are true or false. This is not the task of logic. Nonetheless, it is a question which we should always ask in evaluating arguments. • (2) Whether the relationship between the premises & conclusion of an argument is correct. This is the task of logic. Evaluating deductive arguments - 2

3. Logic ~ evaluating arguments • Example • Amoebas are not plants, because they are capable of motion, and no plants have that capacity. • (this is a valid categorical syllogism) Evaluating deductive arguments - 3

4. Logic ~ evaluating arguments • General evaluative considerations (cont’d) • 2. The difference in evaluating deductive & inductive arguments • validity  refers to the correctness of the relationship between Ps & C in a deductive argument; is no middle ground • strength  refers to the correctness of the relationship between Ps & C in an inductive argument; is a middle ground Evaluating deductive arguments - 4

5. Logic ~ evaluating arguments • Example of the notion of strength & weakness • In England, which imports most of its sucrose [sugar], records of the last 100 years show a steady increase in the per capita consumption of sucrose, from about 20 pounds per year in 1820 to over 100 pounds per year today. (cont’d on next slide) Evaluating deductive arguments - 5

6. Logic ~ evaluating arguments • Example of the notion of strength & weakness (cont’d) • Present consumption of sucrose in the United States is about the same. This represents 15 to 20 per cent of an individual’s caloric requirements. Concomitant with this increased consumption of sucrose has been an almost parallel rise in the prevalence of caries. Evaluating deductive arguments - 6

7. Logic ~ evaluating arguments • Example of the notion of strength & weakness (cont’d) • Conversely, surveys in Europe and Japan demonstrated that caries were dramatically reduced during periods of wartime restrictions of sugar, syrup, and all sugar products.” (E. Newton, “Sugar and Dental Caries; A Review of Human Studies,” Science 217 (1982): 418. Evaluating deductive arguments - 7

8. Logic ~validity • In this section, we shall focus on validity • The notion of validity • Review up to this point • 1. Validity & invalidity apply to deductive arguments only • 2. Validity & invalidity is either/or; there is no middle ground Evaluating deductive arguments - 8

9. Logic ~validity • A third trait of validity • 3. Validity & invalidity are a function of the form of the argument vs content Evaluating deductive arguments - 9

10. Logic ~validity • Example • (1) No fideists are rationalists. • Some theists are fideists. • Therefore, some theists are not rationalists. Evaluating deductive arguments - 10

11. Logic ~validity • The form of this argument • (2) No F are R. • Some T are F. •  Some T are not R. Evaluating deductive arguments - 11

12. Logic ~validity • Can substitute for F, R, & T. • (3) No convicts may vote. • Some lawyers are convicts. •  Therefore, some lawyers are not eligible to vote. • Since (1) has the same form as (3), if (1) is valid, then (3) is valid. Evaluating deductive arguments - 12

13. Logic ~validity, strength & truth • The relationship between validity, strength, & truth for deduction premises validity conclusion true valid necessarily true true invalid may be true or false false valid may be true or false false invalid may be true or false Evaluating deductive arguments - 13

14. Logic ~validity, strength & truth • E.g., of line 2 • Some acids are soluble in water. Some things which are soluble in water are sweet to the taste. Accordingly, some acids are sweet to the taste. • true premises; invalid argument; false conclusion Evaluating deductive arguments - 14

15. Logic ~validity, strength & truth • E.g., of line 3 • Every flower is an animal. Every dog is a flower. Therefore, every dog is an animal. • argument is valid; the premises are false; the conclusion is true Evaluating deductive arguments - 15

16. Logic ~validity, strength & truth • E.g., of line 4 • All flowers are animals. Some animals are plants. Therefore, all flowers are plants. • argument is invalid; the premises are false; the conclusion is true Evaluating deductive arguments - 16

17. Logic ~validity , strength & truth • for induction premises strength conclusion true strong may be true or false true weak may be true or false false strong may be true or false false weak may be true or false Evaluating deductive arguments - 17

18. Logic ~ on truth • On truth • Preliminary clarification • Truth is a property of statements • Validity & invalidity, strength & weakness are properties of arguments Evaluating deductive arguments - 18

19. Logic ~ on truth • Three theories of truth • 1. Correspondence • Some classic defenders: Aristotle (384-322 B.C.), Thomas Aquinas (1225-1274), John Locke (1632-1704) • A statement is true if it agrees with reality. Evaluating deductive arguments - 19

20. Logic ~ on truth • Strength • Works for many everyday experiences • Weaknesses • Does not work well for statements further removed from everyday experience Evaluating deductive arguments - 20

21. Logic ~ on truth • Determining the meaning of correspondence in this context • 2. Coherence theory • Classic defenders: Francis Bradley (1846-1924); Brand Blanshard (1892-1987) • A statement is true if it belongs to a set of coherent statements. Evaluating deductive arguments - 21

22. Logic ~ on truth • Claims common to this theory: • There are no independent facts; all facts are theory-laden • There is no one foundation of knowledge • All knowledge involves a system Evaluating deductive arguments - 22

23. Logic ~ on truth • Two standards for testing the truth of a system (1) comprehensiveness (2) coherence Evaluating deductive arguments - 23

24. Logic ~ on truth • Evaluation of the coherence theory (in general) • Strength • It can handle statements distant from everyday experience, & mathematical, moral, and aesthetic statements Evaluating deductive arguments - 24

25. Logic ~ on truth • Weaknesses • How is the truth of systems established? • By coherence with other systems? The danger of infinite regress? Evaluating deductive arguments - 25

26. Logic ~ on truth • What if a statement which is true in one system conflicts with a statement which is true in another? • 3. The pragmatic theory • Some classic defenders: Charles Sanders Peirce (1838-1914), William James (1842-1910), & John Dewey (1859-1952) Evaluating deductive arguments - 26

27. Logic ~ on truth • A statement is true if it is useful. • Meanings of useful • makes predictions which are confirmed • enables us to function in everyday life • encourages further inquiry • leads to richer & more fulfilling lives Evaluating deductive arguments - 27

28. Logic ~ on truth • Weaknesses • Broadness of the use of “useful.” Truth is relative -- to individuals and communities • There are false statements which are useful. E.G. “Wolves are vicious.” • There seem to be useless truths. Evaluating deductive arguments - 28

29. Logic ~ on truth • Strength • Emphasizes that “truth matters.” To Part II Evaluating deductive arguments - 29