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Confirmation and the ravens paradox 1 Seminar 3: Philosophy of the Sciences Wednesday, 21 September 2011 1.

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Presentation on theme: "Confirmation and the ravens paradox 1 Seminar 3: Philosophy of the Sciences Wednesday, 21 September 2011 1."— Presentation transcript:

1 Confirmation and the ravens paradox 1 Seminar 3: Philosophy of the Sciences Wednesday, 21 September

2 Required readings Peter Godfrey Smith. Theory and Reality. Section (can be downloaded from HKU library) Clark Glymour ‘Why I am not a Bayesian’ (on course website) 2

3 Optional readings Paul Horwich ‘Wittgensteinian Bayesianism’ (on course website) J. A. Cover and Martin Curd ‘Commentary on confirmation and relevance’, section 5.1, pp (on course website) Hawthorne and Fitelson. ‘The paradox of confirmation’, Philosophy Compass pp (can be downloaded from HKU library) 3

4 Tutorials Tutorials will start on this Friday 23 September Class 1: 1 PM - 2 PM seminar room 305 Class 2: 4 PM – 5 PM seminar room 305 Required reading: ‘The Problem of Induction’, Section I, Chapter 7 of Richard Feldman’s book Epistemology pp (on course website) Required reading and seminar handouts must be brought along to tutorials 4

5 Questions to be addressed Q1) What is it for evidence E to confirm hypothesis H (that is, what is it for E to be evidence in favour of H)? Q2) Which propositions confirm which propositions? Q3) Which propositions do scientists and ordinary people take to confirm which propositions? Note: Skeptics like Hume might still be interested in Q3 5

6 Instances of Q2 Does a being a black raven confirm all ravens are black? Does a being a white shoe confirm all ravens are black? Does there was scratching noises coming from the cupboard last night and the cheese in the cupboard has now disappeared confirm the cheese was eaten by a mouse? Do our data concerning changes in temperature and climate confirm the theory of man-made global warming? 6

7 The instantial theory of confirmation (ITC) For any predicates F and G, and any name a, i) Fa.Ga confirms  x(Fx  Gx) (All Fs are Gs); and ii) Fa.~Ga disconfirms  x(Fx  Gx). Def: ‘.’ means ‘and’ Problem 1 with ITC: There is no obvious way to extend ITC to deal with plausible cases of confirmation like c) and d) described on slide 6. 7

8 Problem 2: The ravens paradox (Carl Hempel) Let ‘R’ symbolise ‘is a raven’, and ‘B’ symbolise ‘is black’. (1)By ITC, ~Ba.~Ra confirms  x(~Bx  ~Rx) (2)  x(~Bx  ~Rx) is necessarily equivalent to  x(Rx  Bx) (3) By (1), (2) and (EQ), ~Ba.~Ra confirms  x(Rx  Bx) 8

9 The ravens paradox (cont) (EQ) If E confirms H1, and H1 is necessarily equivalent to H2, then E confirms H2 (PC) therefore follows from (ITC) and (EQ). But how can a white shoe provide evidence that all ravens are black?? (PC) That a is non-black and non-raven confirms that all ravens are black 9

10 A response to the paradox Since ‘(  xFx).(  x(Fx  Gx))’ is a better symbolisation of ‘All Fs are Gs’ than ‘  x(Fx  Gx)’, ITC should be replaced with ITC*. ITC*) For any predicates F and G, and any name a, i) Fa.Ga confirms (  xFx).(  x(Fx  Gx)); and ii) Fa.~Ga disconfirms (  xFx).(  x(Fx  Gx)). 10

11 A problem with this response Given (SPC), (ITC*) entails (PC). (SPC) If E confirms H1, and H1 entails H2, then E confirms H2. Argument: (1)By ITC*, ~Ba.~Ra confirms (  x~Bx).(  x(~Bx  ~Rx)) (2)(  x~Bx).(  x(~Bx  ~Rx)) entails  x(Rx  Bx) (3) By (1), (2) and (SPC), ~Ba.~Ra confirms  x(Rx  Bx) 11

12 Hempel’s response to the paradox (PC) is true. It seems false because a) We falsely think that ‘  x(Rx  Bx)’ is only about ravens, when in fact it is about all objects, as it is reformulation as ‘  x(~Rx v Bx)’ reveals. b) We falsely think that PC is false, because we fail to distinguish it from the false PC*. (PC*) That a is non-black and non-raven confirms that all ravens are black, given the background information that a is a non-raven. 12

13 Two kinds of confirmation relation 3-place: E confirms H relative to background knowledge K 2-place: E confirms H absolutely Connection between them: E confirms H absolutely iff E confirms H relative to no information (or relative to a logical truth T) ITC is intended as a theory of absolute confirmation 13

14 The hypothetico-deductive theory of confirmation (HDT) i)E confirms H if E can be divided into two parts, E1 and E2, such that a) E1 does not entail E2, but b) the conjunction of H and E1 does entail E2. ii)E disconfirms H if E entails ~H iii)Otherwise E neither confirms or disconfirms H In favour of HDT: HDT fits many episodes in the history of science well. 14

15 Problems with HDT Problem 1: HDT entails PC, and hence faces the ravens paradox Problem 2: HDT cannot account of confirmation of statistical theories such as the hypothesis that anyone who smokes has a 25% chance of developing lung cancer. 15

16 Problem 3: Irrelevant conjunction Suppose evidence E, made up of E1 and E2, is such that i) E1 does not entail E2, but ii) H.E1 does entail E2. Then H.S.E1 entails E2, where S is any hypothesis at all. Hence, according to HDT, E confirms H.S. Moreover, by SPC, E confirms S. But S can be anything at all! 16

17 The probability raising theory of confirmation PRT for absolute confirmation: i)E confirms H iff P(H|E) > P(H) ii)E disconfirms H iff P(H|E) < P(H) where ‘P(H)’ means ‘the probability of H’, and ‘P(H|E)’ means ‘the probability of H given E’. 17

18 The probability raising theory of confirmation (cont) PRT for relative confirmation: i)E confirms H relative to background knowledge K iff P(H|E.B) > P(H|K) ii)E disconfirms H relative to background knowledge K iff P(H|E.B) < P(H|K) 18

19 Quantitative probability raising theories of confirmation Def: c(H,E,K) = the degree to which E confirms H relative to background knowledge K A popular account of c among PRT theorists: D) c(H,E,K) = P(H|E.K) - P(H|K) 19

20 Good’s response to the raven paradox Good’s claim: Whether E=Ra.Ba confirms H=  x(Rx  Bx) relative to K depends on what the background knowledge K is. Example: E won’t confirm H if K is the knowledge that either i)There are 100 black ravens, no non-black ravens and 1 million other birds ii)There are 1000 black ravens, 1 white raven, and 1 million other birds 20

21 Good’s example E won’t confirm H if K is the knowledge that either i)There are 100 black ravens, no non-black ravens and 1 million other birds ii)There are 1000 black ravens, 1 white raven, and 1 million other birds In this case P(E|H.K) < P(E|~H.K), from which it can be proved that P(H|E.K) < P(H|K). 21

22 Good on absolute confirmation Good also claimed that it might be that Ra.Ba fails to confirm  x(Rx  Bx) absolutely. Discuss unicorn case. 22

23 The standard Bayesian strategy to solve the ravens paradox Show that given plausible assumptions about our background knowledge, Ra.Ba confirms  x(Rx  Bx) relative to K more than ~Ra.~Ba. The result if established can then be used to explain why (PC) seems false. 23

24 Hawthorne and Fitelson’s attempt Given the assumptions about K given by (K-ass), H+F show that the following theorem holds. K-ass: i) P(H|Ba.Ra.K), P(H|~Ba.~Ra.K), and P(~Ba.Ra|K) aren’t 0 or 1; and ii) P(~Ba|K) > P(Ra|K). Theorem: If P(H|Ra.K) ≥ P(H|~Ba.K), then P(H|Ba.Ra.K) > P(H|~Ba.~Ra.K). 24


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