Philosophy 170

1. Philosophy 170 Introduction to Logic

2. Chapter 1:Informal Introduction

3. Logic is… • The study of argument • The study of criteria for distinguishing successful from unsuccessful arguments and the study of methods for applying those criteria • An argument is a set of statements, some of which—the premises—are supposed to support, or give reasons for, the remaining statement—the conclusion • In a successful argument the premisesgenuinely support the conclusion • ‘genuine support’ requires the probableor guaranteed preservation of truth from premises to conclusion • The study of related properties such as consistency, logical truth, etc. • The key to a world of wonder

4. Logic is not… • Logic is not the study of persuasion and manipulative rhetorical devices • ‘successful argument’ does not mean persuasive argument • Human fallibility and manipulative rhetoric lead people to • accept poor reasoning • reject good reasoning • Remember, in a successful argument if the premises are true, then the conclusion is either guaranteed to be true or likely to be true

5. Why Study Logic? • Intrinsic value • Enjoyment of learning • Enjoyment of abstract structures and analytic elegance • Enjoyment of puzzles and figuring things out • Instrumental value • Improve abstract, critical, and analytic reasoning • Increase the number of tools in your critical thinking “toolkit” • Improve writing, reading, speaking skills • Become a better thinker/knower • Become a more independent thinker • Become the life of the party

6. Some Definitions: Statement:A statement is a declarative sentence; a sentence which attempts to state a fact—as opposed to a question, command, exclamation, etc. Argument:an argument is a (finite) set of statements, some of which—the premises—are supposed to support, or give reasons for, the remaining statement—the conclusion Logic:Logic is the study of • criteria for distinguishing successful from unsuccessful argument, • methods for applying those criteria, and • related properties of statements such as implication, equivalence, logical truth, consistency, etc. Truth Value:The truth value of a statement is just its truth or falsehood; we assume that every statement has either the truth value true, or the truth value false, but not both

7. An Example Argument • Socrates is mortal, for all humans are mortal, and Socrates is human • Given that Socrates is human, Socrates is mortal; since all humans are mortal • All Humans are mortal, Socrates is human; therefore Socrates is mortal

8. Premise and Conclusion Indicators Premise Indicators:as, since, for, because, given that, for the reason that, inasmuch as Conclusion Indicators:therefore, hence, thus, so, we may infer, consequently, it follows that

9. Premise 1 Premise 2  Premise n Conclusion All humans are mortal Socrates is human Socrates is mortal Standard Form

10. Argument Form and Instance Argument Form and Instance:An argument form (or schema) is the framework of an argument which results when certain portions of the component sentences are replaced by blanks, schematic letters, or other symbols. An argument instance is what results when the blanks in a form are appropriately filled in

11. Form: All F are G x is F x is G Instances: All humans are mortal Socrates is human Socrates is mortal All monsters are furry Grover is a monster Grover is furry Form and Instance

12. Two Types of Criteria for Successful Arguments • Deductive • Inductive • These criteria have some things in common, but will turn out to be importantly different • The distinction is NOT • Deductive = general to specific • Inductive = specific to general • THE ABOVE IS INCORRECT • The distinction will involve the nature of the link between premises and conclusion • This is best illustrated…

13. All whales are mammals All mammals are air-breathers All whales are air-breathers “Good” or “Bad”? Argument 1A T T T All Premises True Conclusion True F1 G1

14. Argument 1B All whales are fish All fish are air-breathers All whales are air-breathers “Good” or “Bad”? F F T At least One Premise False Conclusion True F1 G1

15. All whales are reptiles All reptiles are birds All whales are birds “Good” or “Bad”? Argument 1D F F F At least One Premise False Conclusion False F1 G1

16. Form 1 All F are G All G are H All F are H F2 G1

17. Conclusion True Conclusion False F1 G2

18. Argument 2A Some animals are frogs Some animals are tree-climbers Some frogs are tree-climbers “Good” or “Bad”? T T T All Premises True Conclusion True F2 G2

19. Argument 2B Some fish are frogs Some fish are tree-climbers Some frogs are tree-climbers “Good” or “Bad”? F F T At least One Premise False Conclusion True F2 G2

20. Argument 2D Some fish are frogs Some fish are birds Some frogs are birds “Good” or “Bad”? F F F At least One Premise False Conclusion False F2 G2

21. Argument 2C Some animals are frogs Some animals are birds Some frogs are birds “Good” or “Bad”? T T F All Premises are True Conclusion False F2 G2

22. Form 2 Some F are G Some F are H Some G are H F1 G2

23. Conclusion True Conclusion False F2 G1

24. Evaluating Deductive Arguments Deductive Validity, Invalidity:An argument (form) is deductively valid iff* it is NOT possible for ALL the premises to be true AND the conclusion false, it is deductively invalid iff it is not valid Soundness:An argument is sound iff it is deductivelyvalid AND all its premises are true * ‘iff’ is short for ‘if and only if’

25. Conclusion True Conclusion False F1 G2

26. Conclusion True Conclusion False F2 G1

27. Form 1 All F are G All G are H All F are H Valid Form Form 2 Some F are G Some F are H Some G are H Invalid Form Argument Forms 1 & 2

28. Some Points about Validity • Validity a question of Truth Preservation • It is a matter of Form • Thus an argument form is valid (invalid), and any instance of that form is valid (invalid) • It has nothing to do with actual truth values of the sentences involved* • True premises and true conclusion are neither necessary nor sufficient for validity (see 1B, 1D, and 2A) *Except for counterexamples…

29. Counterexamples and Invalidity Counterexample:A counterexample to an argument (form) is an instance of exactly the same form having all true premises and a false conclusion. Production of a counterexample shows that the argument form and all instances thereof are invalid. • This is the ONLY time actual truth values are relevant • If all premises are true and the conclusion is false, that instance, that form, and any other instance of that form are invalid

30. Counterexamples and Invalidity • We can see that a particular argument, an argument form, and all instances of that form are invalid by either: • Offering a counterexample, or • Consistently imagining that all the premises are true and the conclusion is false • Failure to do one of the above shows nothing, however, because it may be just our lack of creativity which prevents us finding a counterexample or imagining the appropriate situation

31. Soundness • An argument is sound iff it is deductively valid and all the premises are true • Unlike validity, soundness does have to do with the actual truth values of the premises • Soundness is only an issue when the argument is valid • Unsound arguments will not convince a worthy opponent • Determining soundness is outside the bounds of logic, it requires non-logical investigation

32. Evaluating Deductive Arguments

33. Invalid but still “good”? There are 4 Jacks in this standard deck of 52 cards The deck has been shuffled The next card drawn will not be a Jack Most Rottweilers have docked tails Ralphie is a Rottweiler Ralphie has a docked tail

34. Evaluating Inductive Arguments Inductive Strength:An argument is inductively strong to the degree to which the premises provide evidence to make the truth of the conclusion plausible or probable. If an argument is not strong, it is weak. Cogency:An argument is cogent iff it is inductively strong AND all the premises are true

35. A1 is F A2 is F  An is F All As (or the next A) are/will be F All 57 trout caught in Jacob’s Creekwere infected with the RGH virus All trout (or the next trout caught; or x% of trout) in Jacob’s Creek will be infected with the RGH virus Induction by Enumeration • The As are the sample—the observed instances or examples; • F is the target property

36. A is F, G, H B is F, G, H, and I A is I My car is a 1999 Toyota Camry Sue’s car is a 1999 Toyota Camry and gets over 30 mpg My car will get over 30 mpg Argument by Analogy • F, G, H are the similarities, I is the target property • The comparison base, B, may be an individual or a group

37. Some Rules of Thumb for Enumerations/Analogies • The larger the sample size or comparison base group, the stronger the argument • The narrower or more conservative the conclusion, the stronger the argument • The greater the number of (relevant) similarities, the stronger the argument • The fewer the number of (relevant) dissimilarities, the stronger the argument

38. Inductive Strength Not a Matter of Form The 12,700 days since my birth have all been days on which I did not die So I will not die today. Indeed, I’ll never die! I like peanuts, am bigger than a breadbox, and have two ears Bingo the elephant likes peanuts, is bigger than a breadbox, has two ears, and has a trunk I have a trunk

39. Validity vs. Strength • Unlike deductive validity, inductive strength is a matter of degree, not an all-or-nothing, on/off switch • Unlike deductive validity, inductive strength is not a matter of form • Unlike deductive validity, additional information is relevant to the assessment of strength

40. Background Knowledge & Strength • Determining strength of an inductive argument has a lot to do with many unstated background assumptions, e.g.: • Relevance of similarities and dissimilarities • Nature and selection of the sample group • Stability of relevant but unstated conditions • It also has to do with the availability of further evidence, thus • Unlike with validity, additional premises (new evidence, change in background assumptions) can increase or decrease the strength of the argument

41. Abduction Abduction:Abduction or abductive reasoning,also known as inference to the best explanation is a category of reasoning subject to inductive criteria in which the conclusion is supposed to explain the premises

42. It is 5pm on Monday The mail has not come The mail carrier is almost never late It must be a holiday I see paw prints on the hood and roof of my car There are fur balls in the corner There are mice guts under the car The garage door was left open The cat slept in the garage Examples

43. About Abduction • The more data accounted for the better the explanation • The better the explanation coheres with already confirmed theory, the better it is • The more new data successfully predicted, the better the explanation • So, again, background assumptions are relevant • There is almost always more evidence available, and it might lead to a reassessment of the inference/argument • Exactly what is meant by “best” is not entirely clear

44. Evaluating Inductive Arguments

45. Some Logical Properties of Statements • Alice fell down a rabbit hole • If Alice fell down a rabbit hole, then it’s not the case that Alice didn’t fall down a rabbit hole • Alice fell down a rabbit hole and did not fall down a rabbit hole

46. Properties of Individual Statements Logically True:A statement is logically true if and only if it is not possible for the statement to be false Logically False:A statement is a logically false if and only if it is not possible for the statement to be true Logically Contingent:A statement is a logically contingent if and only if it is neither logically true nor logically false; i.e., it is both possible for the statement to be true, and possible for the statement to be false

47. Properties of Pairs of Statements • Alice fell down a rabbit hole • Alice drinks from the bottle labeled “DRINK ME” • If Alice doesn’t drink from the bottle labeled “DRINK ME”, then she won’t fit through the door • To fit through the door, Alice must drink from the bottle labeled “DRINK ME” • The White Rabbit does not speak and carry a watch • The White Rabbit speaks and carries a watch

48. Pairs of Statements Logically Equivalent:A pair of statements is logically equivalent if and only if it is not possible for the statements to have different truth values Logically Contradictory:A pair of statements is logically contradictory if and only if it is not possible for the statements to have the same truth values

49. Sets of Statements { Alice eats the cake marked “EAT ME”, Alice grows large enough to get the key, Alice is again too large to fit through the door } { Alice eats the cake marked “EAT ME”, If Alice eats the cake marked “EAT ME” she will grow too large to fit through the door,Alice fits through the door }

50. Sets of Statements Logically Consistent:A set of statements is logically consistent if and only if it is possible for all the statements to be true Logically Inconsistent:A set of statements is logically inconsistent if and only if it is not possible for all the statements to be true