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Why science is important


Thomas Bayes and Bayesian probabilities
Richard Gregory Scientist
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In the 18th century, there was a very interesting non-conformist priest called Bayes, Thomas Bayes, B-A-Y-E-S, who was an amateur mathematician, and he came up with a formula for relating the probability of the evidence to the probability of the previous knowledge. So he called this priors, as previous knowledge, posteriori, the evidence added on by the particular observation and the formula relates to these in a mathematical way. This became important for, as I understand it, for decision-making in economics and economists have mathematical models for making decisions based on Bayesian statistics, which incidentally he never published in his own lifetime. A friend found this paper after he was dead, you know, put it to the Royal Society and then it was published and it’s now one of the most famous papers, I suppose, certainly in that area of science that there is. Incredibly important, actually, because it’s quantified what was a rather nebulous idea, I think, in a way, following on from Ockham, you could say. Anyway, what I find interesting is looking at the Bayesian formulation both for science, that is hypothesis in science, and then for perceptions because I’ve always thought that perceptions are hypothesis. I think the brain is making a guess at what is out there on essentially inadequate evidence, it’s doing the best it can, and it’s always got the probability of the available evidence in terms of the hypothesis to be tested, and, of course, the more unlikely the hypothesis, the more strong you need the probabilities of the evidence in order to modify it or change it or throw it out. And it may throw it out wrongly; it may pop back again later. So when you get these ambiguities in perception, like the duck rabbit, that is entertaining alternatives because there’s no clear Bayesian imbalance in the probabilities between different hypothesis so it oscillates, it modifies the system such that the duck is seen the rabbit is seen. Now, what I like about the hollow face illusion is this. This is a hollow mould of a face, so the nose is actually sticking in instead of out but, blow me down, you look at this thing from a bit of a distance and it absolutely looks like an ordinary face with the nose sticking it, convex. Why? Because of Bayesian probabilities. The brain says to itself, that’s ridiculous, this thing is clearly a face, it’s got eyes, it’s got a mouth, etc, how on earth can the thing be hollow when it’s a face because faces are not hollow things. So the prior probability dominates, throws out the evidence of the senses, the eye, and the illusion dominates. So it’s not like the duck rabbit, this could be either. The illusion, the false possibility, dominates over the truth because it is unlikely, from past experience, that the face is hollow. This is the point. Now, this is the sort of Bayesian strategy and what’s interesting about Bayes is that he actually quantified this, set up a really neat mathematical formulation and so you could put this into a computer, if you like, you can treat it objectively, explicitly, with the methods of science, and yet, looking at the other side of it, artists are playing about with this all the time, you know, presenting shapes which might be one thing, might be another, they sort of morph in the brain from one thing to another so the artist plays about with what in science you can define in terms of Bayesian statistics. To add just a little bit to this, the statistics here are subjective, the probabilities are subjective, they’re in your brain. The reality provides relative frequencies of events like how often it rains on a Thursday, or how often it rains in summer or winter, that’s objective, but the probability assigned to that is given by the creative, intelligent, or not so intelligent, brain. So you’ve got this wonderful interplay between objectivity of events, frequencies of events, and the subjective assessment of the probabilities which can be wildly different from the ratio of frequencies in the external world objectively so that the subject and the object can be way apart and artists and scientists, I think, are playing with opposite extremes of this.

The late British psychologist and Emeritus Professor of Neuropsychology at the University of Bristol, Richard Gregory (1923-2010), is well known for his work on perception, the psychology of seeing and his love of puns. In 1978 he founded The Exploratory, an applied science centre in Bristol – the first of its kind in the UK. He also designed and directed the Special Senses Laboratory at Cambridge which worked on the perceptual problems of astronauts, and published many books including 'The Oxford Companion to the Mind', 'Eye and Brain' and 'Mind in Science'.

Listeners: Adam Hart-Davis Sally Duensing

Born on 4 July 1943, Adam Hart-Davis is a freelance photographer, writer, and broadcaster. He has won various awards for both television and radio. Before presenting, Adam spent 5 years in publishing and 17 years at Yorkshire Television, as researcher and then producer of such series as Scientific Eye and Arthur C Clarke's World of Strange Powers. He has read several books, and written about 25. His latest books are Why does a ball bounce?, Taking the piss, Just another day, and The cosmos: a beginner's guide. He has written numerous newspaper and magazine articles. He is a keen supporter of the charities WaterAid, Practical Action, Sustrans, and the Joliba Trust. A Companion of the Institution of Lighting Engineers, an Honorary Member of the British Toilet Association, an Honorary Fellow of the Royal Photographic Society, the Royal Society of Chemistry, the Society of Dyers and Colourists, and Merton College Oxford, and patron of a dozen charitable organizations, Adam has collected thirteen honorary doctorates, The Horace Hockley Award from the Institute of Scientific and Technical Communicators, a Medal from the Royal Academy of Engineering, the Sir Henry Royce Memorial Medal from the Institute of Incorporated Engineers, and the 1999 Gerald Frewer memorial trophy of the Council of Engineering Designers. He has no car, but three cycles, which he rides slowly but with enthusiasm.

Sally Duensing currently is involved in perception exhibition work and research on science and society dialogue programmes and is working with informal learning research graduate students and post-docs at King's College, London. In 2000 she held the Collier Chair, a one-year invited professorship in the Public Understanding of Science at the University of Bristol, England. Prior to this, for over 20 years she was at the Exploratorium, a highly interactive museum of science, art and perception in San Francisco where she directed a variety of exhibition projects primarily in fields of perception and cognition including a large exhibition on biological, cognitive and cultural aspects of human memory.

Duration: 5 minutes, 24 seconds

Date story recorded: June 2006

Date story went live: 02 June 2008