Church s s Thesis and Hume s s Problem:

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1 Church s s Thesis and Hume s s Problem: Justification as Performance Kevin T. Kelly Department of Philosophy Carnegie Mellon University kk3n@andrew.cmu.edu

2 Further Reading (With C. Glymour) Why You'll Never Know Whether Roger Penrose is a Computer, Behavioral and Brain Sciences, 13: The Logic of Reliable Inquiry, Oxford: Oxford University Press, (with O. Schulte) Church's Thesis and Hume's Problem, in Logic and Scientific Methods, M. L. Dalla Chiara, et al., eds. Dordrecht: Kluwer, (with O. Schulte and C. Juhl) Learning Theory and the Philosophy of Science, Philosophy of Science 64: The Logic of Success, British Journal for the Philosophy of Science, 51:2000. Uncomputability: The Problem of Induction Internalized, Theoretical Computer Science 317: Computability and Learnibility, Textbook manuscript, 2005.

3 Fateful First Cut Relations of Ideas Analytic Certain A Priori Philosophy of Math Proofs and algorithms Truth-finding Computability Matters of fact Synthetic Uncertain A Posteriori Philosophy of Science Confirmation Rationality Probability

4 Painful Progress Relations of Ideas Analytic Certain A Priori Philosophy of Math Proofs and algorithms Truth-finding Computability Matters of fact Synthetic Uncertain A Posteriori Philosophy of Science Confirmation Rationality Probability

5 Slight Rearrangement Philosophy of Math Philosophy of Science Certain A Priori Uncertain A Posteriori Proofs and algorithms Truth-finding Computability Confirmation Rationality Probability

6 Philosophy of Science 101 Formal Questions Scientific Questions Unverifiable Verifiable The sun will never stop rising The given number is even

7 Wrong Twice! Formal Questions Unverifiable The given computation will never halt Verifiable Scientific Questions Unverifiable The sun will never stop rising Verifiable The given number is even The next emerald is green

8 Classical Excuse Relations of Ideas Matters of fact Completed analysis Rational Animal Unbounded experience Rational??? Always black? Reason Observation

9 But Maybe Relations of Ideas Matters of fact Unbounded analysis Unbounded experience?? Rational??? Always Never white? black? Reason Observation

10 Kant Synthetic A Priori A Posteriori Unbounded experience temporal intuition 3+2 = 5??? Always Never white? black? Intuition Observation

11 Oops! Synthetic A Priori A Posteriori spatial intuition Unbounded experience?? Never cross??? Always Never white? black? Intuition Observation

12 After Turing Formal Problems Empirical Problems Unbounded computation Input n Unbounded reality???? Never halts? Always Never white? black? Computer Observation

13 Bad First Cut Formal Questions Unverifiable The given computation will never halt Verifiable Scientific Questions Unverifiable The sun will never stop rising Verifiable The given number is even The next emerald is green

14 Good First Cut Formal Questions Unverifiable The given computation will never halt Verifiable Scientific Questions Unverifiable The sun will never stop rising Verifiable The given number is even The next emerald is green

15 Computational Epistemology For formal and empirical questions: Justification = truth-finding performance Find the truth in the best possible sense And then as efficiently as possible.

16 Verification as Support 1.0 Deductive verification.5 Inductive verification

17 Paradigms Evolve Objective support by evidence.

18 Paradigms Evolve Objective support by evidence. Coherent personal opinion. Rational update.

19 Paradigms Evolve Objective support by evidence. Coherent personal opinion. Rational update.

20 But Leading Metaphors Matter Myopic Bedrock = rationality intuitions Success = being rational Truth-finding performance deferred or ignored Problem complexity ignored Little resemblance to computability

21 Verification as Halting Conclusion Data

22 Verification as Halting Conclusion Data Yes!

23 Verification as Halting Conclusion Data

24 Verification as Halting Conclusion Data

25 Verification as Halting Conclusion Data Bang!

26 Verification as Halting Conclusion Data Can t take it back.

27 Verifiability Conclusion true Yes! Conclusion false

28 Laws are Not Verifiable Always green?

29 Laws are Not Verifiable Always green?

30 Laws are Not Verifiable Always green?

31 Laws are Not Verifiable Always green?

32 Laws are Not Verifiable Always green? You must halt with yes if it s true!

33 Laws are Not Verifiable Always green? You must halt with yes if it s true!

34 Laws are Not Verifiable Always green? Yes!

35 Laws are Not Verifiable Always green?

36 Laws are Not Verifiable Always green?

37 A Purely Formal Problem Never shoots?

38 Analogy? I ain t a shootin, yeller belly! Never shoots?

39 Apparently Not Game s up, J.R.! Yer program s showin! Never shoots? 666

40 Apparently Not Now shaddup! I m calculatin a-preeorey-like what yer gonna do. Never shoots? 666

41 Apparently Not?!!! Never shoots?

42 But Then Again I got it! Two kin play that opry oary game! Never shoots?

43 But Then Again I won t shoot til I see him shoot in this here opry oary 666 sim-yooo-laytion! Never shoots?

44 But Then Again I ain t a shootin Never shoots?

45 But Then Again I still ain t a shootin Never shoots?

46 But Then Again You check that there opry oary all you want Never shoots?

47 But Then Again An all you ll see is I ain t a-shootin Never shoots?

48 But Then Again Aw, shucks, J.R. I reckon You ain t never gonna shoot Aw, shucks, J.R. I reckon you ain t never gonna shoot. Never shoots?

49 But Then Again Never shoots? 666 Bang! 23455

50 But Then Again Never shoots?

51 But Then Again See ya in the grand ol opry oary, sucker! Never shoots?

52 But Then Again Take that! Never shoots? Bang!

53 But Then Again Never shoots?

54 But Then Again ROBO SHERIFF Fooled A priori Never shoots?

55 Rice-Shapiro Theorem Whatever a computable cognitive scientist could verify about an arbitrary computer s input-output behavior by formally analyzing its program Q QDoc

56 Rice-Shapiro Theorem could also have been verified by a behaviorist computer who empirically studies only the arbitrary computer s inputoutput behavior! Q QDoc

57 Uncomputable Induction H N Yes! Will see a non-halting machine Verifiable by ideal agent.

58 Uncomputable Induction H N? Will see a non-halting machine Q Not verifiable by computable agent.

59 Similarity Halting Problem Unverifiable Demon can fool computable agent Answer runs beyond formal experience Universal Law Unverifiable Demon can fool ideal agent Answer runs beyond empirical experience

60 A Difference Halting Problem Input ends Can handle some examples a priori Universal Law Input never ends Problem of induction Internal problem of induction eventually catches up!

61 Give and Take Formal input Jaw Breaker Fits in mouth but may never melt May never swallow Empirical input Noodle May never fit in mouth May never swallow...

62 Another Difference Formal Science Incompleteness Empirical Science Problem of induction

63 Not Necessarily Formal Science Incompleteness Add more powerful axiom Who knows if new axiom is consistent with old? Assume it is, keep checking, and hope Empirical Science Problem of induction Conjecture a theory Who knows if theory is consistent with data? Assume it is, keep checking, and hope

64 Thesis The problem of induction and the problem of uncomputability are essentially similar. So the two should be understood similarly. From the ground upward.

65 Unverifiability Confirmation Support Coherence Rational update, etc. A House Divided Uncomputability. Suck it up Logic vs. engineering Try to approximate, etc.?!

66 Unified Approach: Gun Control Find the truth in the best feasible sense. So drop the halting requirement if infeasible. Q

67 Start with Problems,, Not Methods Partition Q over set K of inputs A B C D Finite input Infinite input

68 Convergence Convergence with halting????a Convergence in limit (in limit) No more revisions??? A?? B C? C C... revisions

69 Success Converge to right answer A B C D Don t converge to wrong answer

70 Special Cases Verify A Refute A A B A B Decide A A B

71 Special Cases Compute partial φ φ φ = 0 φ = 1 φ = 2... Dom(φ) Theory selection Μ 0 Μ 1 Μ 2...

72 Solutions and Optimality Solution: method that succeeds on each input. Optimal solution: solves in best possible sense Solvable problem: has solution. Problem complexity: best sense of solvability

73 A B Verifiability convergence with 1 revision ending with no. Refutability convergence with 1 revision ending with yes. Decidability convergence with 0 revisions.

74 A B n-refutation = n+1 rev ending with -A n-verification = n+1 rev ending with A n-decision = n rev

75 ... Underdetermination and open Lim-ver Lim-ref Lim-dec... 1-ver 1-ref 1-dec 0-ver 0-ref 0-dec clopen or closed

76 ... Lim-ver Lim-ref Uncomputability and r.e. Lim-dec... 1-ver 1-ref 1-dec 0-ver 0-ref 0-dec decidable or Co-r.e.

77 ... Uncomputability + Underdetermination and r.e. Lim-ver Lim-ref Lim-dec... 1-ver 1-ref 1-dec 0-ver 0-ref 0-dec decidable or Co-r.e.

78 Ideal empirical Effective empirical Effective formal

79 Topological invariants Recursive invariants Recursive invariants

80 Lower Bound Combination Lower Bound

81 Empirical Borel Mixed effective Borel Formal arithmetical

82 Empirical Borel Mixed effective Borel Formal arithmetical

83 Infinitely divisible Finitely divisible Matter

84 Lim-ref Infinitely divisible Finitely divisible Lim-ver I say inf when you let me cut fin fin inf fin

85 Lim-ref -Lim-ver Infinitely divisible Finitely divisible Lim-ver -Lim-ref I let you cut when you say fin inf fin inf inf

86 Lim-ref -Lim-ver Infinite domain Finite domain Lim-ver -Lim-ref

87 Lim-ref -Lim-ver Infinite domain Finite domain Lim-ver -Lim-ref I halt on another input each time he says fin in my opry oary simulation of him ganderin at my program

88 Finite divisibility Finite divisibility Finite domain... Ideal empirical Effective empirical Effective formal

89 Computable Uncomputable???. Human subject Cognitive scientist

90 Computable Uncomputable You can verify human computability in the limit UCUCCCCCCCC Human subject Cognitive scientist

91 Computable Uncomputable But only if you aren t computable! UCUCUCCUCCU Human subject Cognitive scientist

92 Ideal Lim-ver Lim-ref Lim-dec

93 We can verify our computability in the limit only if we are not computable! Computable Lim-ver Lim-ref Lim-dec

94 Even assuming computability, you can converge to the true computable behavior only if you are not computable! Total computable functions φ 0 φ 32 φ

95 (with Oliver Schulte) T -T There exists T such that T makes a unique prediction at each stage; The predictions are very uncomputable

96 (with Oliver Schulte) T -T There exists T such that T makes a unique prediction at each stage; The predictions are very uncomputable But T is refutable by a computable method!

97 Ideal rationality No consistent computable convergent method. Exists inconsistent computable refuting method. Exists consistent computable non-convergent method. So a foolish consistency precludes truth-finding!

98 The very idea of insulating deduction from empirical data restricts the truth-finding power of effective science. Empirical data n Yes Theorem Prover? Q

99 The very idea of insulating deduction from empirical data restricts the truth-finding power of effective science. Empirical data Theorem Prover n n Q Yes

100 Q Justification = truth-finding performance. Performance depends on problem complexity. Formal and empirical complexity are similar. Computational epistemology is unified Standard epistemology is divided. Insistence on division weakens truth-finding performance of effective science.

101 Q Standard epistemology Q Computational epistemology

102 I m rational Q Standard epistemology Q Computational epistemology

103 I m rational Q Standard epistemology Q Computational epistemology

104

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