On 6 April 2006 Chris Green wrote [snip]: > I made an obviously jocular reference (note the smiley) to > Bayesianism in discussing the differences between doing history > and doing natural science. Allen took my off-hand remark as an > opportunity to lecture us on the putative glories of Popperian > philosophy of science as compared with Bayesianism > as though either of them hand much relevance to the > practice of historical research).
I did see the smiley when I first read the sentence in question, but I have to admit that its significance eluded me. (I still don't get the joke, but that's probably because I don't know enough about Bayesian probability theory.) But that's really quite irrelevant to my response, as what I went on to consider, in relation to the issue under discussion, was the probabilistic position advanced by Chris in the sentence immediately before the jocular allusion to Bayesianism: >One should conclude (tentatively, of course, but we're all fallibilists >now, yes?) whatever seems to have been most likely, given the general >information one has. I leave to TIPSters to decide whether what I wrote was an examination of this position, with specific reference to the Darwin issue we've been debating, in the light of one very general notion Popper expounded (which one can regard as valuable completely independently of his flawed "refutability" theory of scientific methodology), or a "lecture on the putative glories of Popperian philosophy of science as compared with Bayesianism". And whether the paragraph of exposition by Chris that followed (see below) really addresses the specific arguments I advanced in the context of his contentions about Darwin. Allen Esterson Former lecturer, Science Department Southwark College, London [EMAIL PROTECTED] http://www.esterson.org/ ----------------------------------------------------------------- Thu, 06 Apr 2006 15:40:34 -0400 Author: "Christopher D. Green" <[EMAIL PROTECTED]> Subject: Sir Karl and Rev. Thomas > I made an obviously jocular reference (note the smiley) to Bayesianism > in discussing the differences between doing history and doing natural > science. Allen took my off-hand remark as an opportunity to lecture us > on the putative glories of Popperian philosophy of science as compared > with Bayesianism (as though either of them hand much relevance to the > practice of historical research). First off, no one is REALLY Bayesian > when it comes to history. I don't think I've ever come across anyone > seriously claiming a specific probability with respect to any historical > event. Even if we were to take my little joke seriously, however, Allen > fails to mention that Popper's falsificationism turns out to be nothing > more or less than a limiting special case of Bayesianism -- viz., when > the probability of the data given the theory is zero (p(D|T)=0). In > other words, when the data flatly contradicts the theory, because p(D|T) > appears as a multiplier in the numerator of the Bayesian function, > whatever the value of the other elements in the Bayesian function -- > p(T), p(D) -- the whole thing goes immediately to 0 (i.e., in Popperian > terms, it is falsified). > > What makes the Bayesian approach so much more useful and interesting > than Popper's falsificationism, however, is that it also tells us what > to do when the probability of the data given the theory is NOT 0, but > some number between 0 and 1. For instance, what if I have a theory that > says there is an 80% probablility that airplane will fly over my house > at exactly 2:30 pm (or to make it more "ecologically valid," as they > say, that another hurricane will strike New Orleans this summer). The > Popperian approach is stopped dead in its track by such a probabilistic > prediction (while Popper himself unaccountably says that it's utterly > unimportant how such a prediction turns out). Bayesianism, however, > enables one to generate a new probability for my theory based on my > prediction and the outcome of the corresponding observation. As > confirming evidence mounts, the theory's probability rises (and old > Popperians moan). As disconfirming (but not flatly falsifying) evidence > comes in, the theory's probability goes down. > > Regards, > -- > Christopher D. Green > Department of Psychology > York University > Toronto, ON M3J 1P3 > Canada > > 416-736-5115 ex. 66164 > [EMAIL PROTECTED] > http://www.yorku.ca/christo > ============================= Thu, 6 Apr 2006 03:40:12 -0400 Author: "Allen Esterson" <[EMAIL PROTECTED]> Subject: Re: Evolution in the news > Since the Darwin debate between me and Chris has opened out into a > discussion of general principles, I'd like to add my penny's worth. Chris > says he is arguing on the basis of "what seems most likely", or Bayesian > principles. On the other hand, I've already invoked Popperian principles, > so let's see how all this falls out. > > Jim Clark wrote: > > Chris is willing to infer from a generally well-founded model to the > > specific case of Darwin, whereas Allen would like to see concrete > > evidence with respect to Darwin. > > Chris responded: > > One can only draw the best inferences possible based on the information > > that is availabe. Specific concrete evidence is grand, when it exists. > > When it doesn't, however, that doesn't mean all bets are off. One should > > conclude (tentatively, of course, but we're all fallibilists now, yes?) > > whatever seems to have been most likely, given the general information > > one has. I guess this means I'm fairly Bayesian, even about history! :-) > > Well, let's see how this works in the present case. Here's an example > given by Chris as one more instance that supposedly supports the notion > that Darwin was reluctant to publish his theory because of repercussions > that would in some way disrupt his comfortable social position, perhaps by > his being socially ostracized: > > Note also that (despite his own abandonment of religious conviction) > > he never denounced the Church or its teachings publicly. > > Well, that *seems* plausible. One more instance to add to the others that, > on Chris's argument, increases the probability that his view is the most > likely explanation for the agreed facts about Darwin's tardiness in > publishing his theory. However, let's look at this instance a little more > closely. "Darwin never denounced the Church or its teachings publicly." > But, unlike some of his followers, Darwin was not a polemicist and there > is nothing in his personal history that indicates that he was inclined > towards this course of action. He was simply not interested in having a > confrontation with the Church. Furthermore, even if he *had* had any such > inclination, there's an overwhelming reason why he wouldn't have indulged > it his deep concern for the feelings of his devout wife Emma. So when we > examine the situation more closely in this instance, the "plausibility" > angle begins to look very much more fragile as supporting evidence for the > thesis being proposed. (And I think I showed this was the case in other > instances when I filled in details around the bare bones that Chris > presented.) > > But even if we consider examples where alternative explanations do not so > clearly make the one proposed superfluous, I would argue there's a > fundamental weakness in Chris's position. And here's where I invoke the > ghost of Karl Popper. Chris has effectively admitted that he can't produce > concrete evidence, but he can supply a good many instances for which the > thesis supplies a plausible explanation, and that this makes for a strong > probability that the thesis is valid. What this boils down to is that we > have a theory about Darwin's behaviour, and instances are cited where, if > we assume the theory, the behaviour makes sense. Popper calls this > interpretation in the light of the theory. However, if we start from the > assumption that the theory is true, it is not difficult to find examples > of (in this case) Darwin's behaviour which can be presented as being > plausible supporting evidence *when interpreted in terms of the theory*. > And this can seem quite compelling, especially if, as is so often the case > when a thesis is being pursued, other aspects of the situation which show > that the behaviour in question can be perfectly well explained without > recourse to the theory are omitted. In this way specific examples may be > presented as amounting to, at least in probabilistic terms, confirmatory > evidence in favour of the theory; i.e., Darwin's behaviour "fits" the > theory. Here's what Popper has to say on such situations: "It was > precisely this fact that they always fitted, that they were always > confirmed which in the eyes of their admirers constituted the strongest > argument in favour of these theories [i.e., theories based on > interpretations of behaviour in the light of the theory]. It began to dawn > on me that this apparent strength was in fact their weakness." > > How so? Because (to reiterate) if you look at various situations (e.g., > the fact that Darwin didn't denounce the Church publicly) and interpret > his behaviour *in the light of the theory you have already embraced*, then > you will have no difficulty in finding 'confirmations' of your theory. And > this is especially so, as so often happens, if you focus on your way at > looking at the behaviour in question, and disregard perfectly plausible > alternative explanations. In this way one can build up a battery of > plausible 'confirming' pieces of evidence, even if not a single one is in > itself compelling, or without an alternative explanation. Does the fact > that *numerous* such pieces of 'confirmatory' evidence can be presented > make the theory more probable? I would argue "no", not just because you > are looking at the selected instances of behaviour through spectacles > already coloured by your theory but also because multiple instances of > uncompelling evidence do not add up to strong evidence. One can develop a > strong sense of *conviction* in this way, but I think it is a kind of > spurious conviction. > > With a background of a considerable amount of reading about Darwin over > several decades, there was not a single one of the instances that Chris > presented as evidence for the thesis he was arguing for to which I felt > unable to respond (to coin a phrase -:) ), "I have no need of that > hypothesis" in other words, that Darwin's behaviour could be perfectly > well explained without recourse to the thesis that Chris put forward. > > I don't suppose for one moment that what I have written will change > anyone's mind on the question at issue. In the end it is a matter of > judgement, and I learned long ago to be wary of theories that depend on > evidence interpreted (usually with a fair degree of tunnel vision) in the > light of the theory it purportedly validates, regardless of how > widely-held the theory may be. > > I'm sure it will be argued that we all, inevitably, look at the evidence > through lenses coloured by our own preconceptions, and of course there is > truth in this. But to a fair extent we can endeavour to offset this by > being continuously aware of it, and making efforts to look at the evidence > from different angles. Equally important, we should look closely at the > situation which is being presented as evidence for the theory to see if > information not being provided by the protagonist puts a rather different > complexion on the matter. > > One last point on the case we have been considering. What *did* happen > when Darwin published *Origin of Species* after events gave him a shot in > the arm? Great publicity and controversy, denunciation of the book from > some quarters but no social ostracism or adverse financial > repercussions. His comfortable life went on very much as before in Downe > and his village neighbours didn't shun him on account of the anti-Biblical > implications of his book. Of course that doesn't *prove* that he had not > been needlessly fearing such negative repercussions but it certainly > doesn't help the theory. > > Reference: > Popper, K. (1963 [1957]). "Science: Conjectures and Refutations." In > *Conjectures and Refutations*, London: Routledge and Kegan Paul, pp. > 33-65. > > Allen Esterson > Former lecturer, Science Department > Southwark College, London > [EMAIL PROTECTED] > http://www.esterson.org/ --- You are currently subscribed to tips as: [email protected] To unsubscribe send a blank email to [EMAIL PROTECTED]
