Genes, Romano-British history and bullshit: analysis of a dispute

In July/August 2017 there was a bit of ‘disagreement’ on the social media platform Twitter, with some right-wing users attacking the BBC over an educational cartoon about Roman Britain, because some of the characters were shown with a darker skin than others. Sides were taken in articles and postings by experts and others on the question of ethnic diversity in Roman Britain. This same issue has popped up again on occasions since.

Mary Beard, the classical historian, emphasised what is known about the diversity of the Roman presence in Britain, and came in for considerable abuse from the far-right.There was an intervention on the other side from Nassim Nicholas Taleb, the statistician, who made various assertions based on genetics. It’s not my purpose to discuss these arguments here: it does seem that both sides talked past each other to some extent, especially in regards to the meaning of ‘diversity’. You can find more about this here and here and here (and other articles).

There is just one particular point I am interested in here, and it arises from an intervention on Twitter from Taleb, which I responded to at the time.

@wmarybeard: this is indeed pretty accurate, there’s plenty of firm evidence for ethnic diversity in Roman Britain

@nntaleb: Historians believe their own BS. Where did the subsaharan genes evaporate? NorthAfricans were lightskinned.
Only “Aethiopians”, even then

@nntaleb: We have a clear idea of genetic distributions hence backward composition; genes better statisticians than historian hearsay bullshit

Trying to leave aside the left-right political positions and the racist motivations involved in the wider discussion, this particular argument seems to reflect an idea that supposed ‘hard science’ (in this case, statistical genetics) trumps ‘soft’ (history/archaeology). I do not want to get into discussion of ‘hardness’ and ‘softness’ here, but I shall try to analyse Taleb’s specific argument about genetic distributions.

There are three sets of data in this problem. One is the set of data on the genes of the living population of interest, and another is the set of genetic data on the historic (or prehistoric) population that the living population is being compared with. There is also another set of data: the written records, archaeological finds and all other items that comprise what Taleb dismisses as ‘hearsay bullshit’.

All of these sets have their own specific technical problems. The historical data certainly have problems of interpretation amongst themselves: dating of objects and authorship or precise meaning of documents, for example, may be uncertain.

But collecting genetic data also has considerable technical problems: for example, obtaining reliable data from living populations is much easier than it is from fossils or human remains, as there are typically fewer remains than there are available living humans. Also the DNA from human remains may have degraded over time or been contaminated with other DNA, such as from microbes, and needs careful separation.

There is an additional problem with the data from both living and past populations: we have to be reasonably sure that the sets we have are representative of the populations they are taken from, since in neither case can we analyse the DNA of every individual. The genome of an individual is ‘data’ or an ‘anecdote’ to the same extent as a single archaeological find or written record is. It doesn’t tell us anything about the population until it is put into context with all the other similar data. This is a particular case where statistics is used: it is a mathematical tool to help us make a (probabilistic) estimate of the genetic composition of the population that our data sets come from. This is not an automatic process: it involves making assumptions and using the right statistical method, so it has its own issues and uncertainties.

We can think of the problem we are trying to solve here as a theory: how can we explain the genetic makeup of the modern population in terms of the genetic makeup of the past population at the period of history that is of interest? Looking at it this way, we can see that there is no ‘backward composition’ we can automatically use to derive one from the other, whether it uses statistics or any other mathematical techniques. The genetics of the modern population must depend on its history, as well as the scientific principles of genes. In the time that elapsed between the past and the present populations, we need to know what has happened to the population. Did certain groups migrate in or out of the population, was there mixing of different populations, was the population subjected to ethnic cleansing or genocide? These are questions that are very hard to settle, for example, to what extent did invading Anglo-Saxons displace the British currently settled in what is now England. Historians have good reason to believe that the Anglo-Saxons formed a new ruling class, and that some of the existing British were displaced by migration, but there is still dispute as to what proportions of the original population were killed or displaced.

So clearly we can’t ignore history here. The current genetic composition of a population is a result of both genetics and history. That history is part of the problem to be solved. Some of it will be attested by artefacts and documentation, some of it is purely hypothetical (and might be solved with assistance from the genetics). The point here is that the historical evidence cannot be dismissed: any theory, including reconstructing ancient populations, must reconcile all the relevant evidence, or it is simply inadequate. If it is contradicted by the evidence (and the evidence is not found to be defective), then the theory is false, and needs modification or replacement. The evidence here includes (at least) the available genetic data and the historical evidence, including written records and archaeological finds.

Note that I am not making claims about the actual history of Roman Britain here.  I don’t have the necessary expertise in the technical fields. I am simply trying to analyse the problem, to see why genetics is not in itself adequate to the problem, and the history cannot be dismissed. Solving the problem necessarily needs the technical expertise of the geneticists, historians and any others with relevant knowledge.

To see why the history is essential, consider the following scenarios (not an exhaustive list) that might happen to a population of interest:

  1. The population remains isolated from any other.
  2. A small number of immigrants arrives, they seize power and become the ruling class, and eventually merge with the main population through interbreeding.
  3. A large number of immigrants arrives and merges with the main population through interbreeding.
  4. A foreign power occupies the country, using troops from other populations, but remains largely separate from the main population. In the end it (largely) leaves.

All these scenarios will leave both different genetic traces and historical and archaeological data. Likely, given the inadequacy of the evidence, the problem will never be finally settled, as there will always be anomalies, gaps in the data and unsolved questions.


Statistics is a branch of mathematics. Strictly speaking, it is fit only for analysing distributions of pure numbers. Whenever statistics is used as a mathematical tool in solving questions about the real world, other restrictions apply. We are then not dealing with pure numbers but with physical entities (and the concepts we use to understand those entities). In the particular problem above, these entities include people and their genes. In the physical world, these entities are subject to other principles, including the human lifecycle and the physical processes of genetic combination.

Sadly, statisticians, however good they are at statistics, can lose sight of this fact, and claim authority for statistics that is simply not justified. A very good example of this is in the supposed debate over human-caused global warming, where statisticians have weighed in on the ‘sceptic’ side with statistical analyses that simply ignore the laws of physics. One statistician has called this ‘mathturbation‘. Climate measurements are not simply numbers, they are properties (such as temperature or carbon dioxide concentration) of physical entities such as air or oceanic water, and our theories about them are part of physics.

In trying to understand things that happen in the real world, outside mathermatical textbooks, you can’t ignore the technical experts and their knowledge: statistics may prove them wrong, but only when correctly applied to the subject data.

Nassim Nicholas Taleb

The involvement of Taleb in this debate was very strange. He seemed to glory in taking the part of extreme right-wing participants in (sometimes vicious) attacks on historians on Twitter. He derided Mary Beard’s academic credentials, and frequently calls experts in other fields ‘bullshitters’. Yet his claim about genes and statistics reproduced above is bullshit, where bullshit is the term for not actually lying, but giving the impression of having knowledge he didn’t actually have.

Taleb’s background is as a statistician and a trader on financial markets. Financial markets are about the nearest thing in the real world to pure numbers, and mistakenly thinking that climate data are in some way similar to markets has led many people into error. Perhaps this confusion applies to other areas as well.

Taleb also seems to have a very thin skin, and has blocked me, and apparently many others who disagreed with him, on Twitter.

Rational or Irrational, or neither?

[BPSDB] [An edited version of a contribution to the Critical Cafe, supposedly a forum for discussing issues related to Karl Popper’s critical rationalism (in fact, occasionally it is). I was alleged, by another contributor, to believe that people are “irrational”. This person himself, If I understand correctly, holds the position – which underlies much of economic theory – that people are “rational” agents pursuing their own self-interest. This short essay is an attempt to clarify my own position.]

I take ‘rational’ to refer to the use of the best available information (including techniques, reasoning etc.) in achieving an aim, solving a problem or acquiring what one needs or desires. ‘Irrational’ presumably would involve not choosing to do this, and it’s hard to understand why anyone would behave in this way.

One could, of course, rationally act on false information that one believes to be correct, perhaps using a flawed argument, and so come to an unfortunate result.  Many errors are the result of cognitive illusions, such as flawed interpretation of probabilities. These arise from the evolved structure of our minds, applied to situations for which evolution has not prepared us. Rationality would involve the application of techniques to overcome these cognitive errors.

Most of what you and I do is entirely out of our conscious control most of the time. This is highly desirable, because if we had to devote mental capacity to rational debate over every single action, we would be immobilised. There are instinctive actions, for example, ducking to avoid a missile, which we have acquired in our evolutionary history because they enabled our survival. These are neither rational nor irrational.

There are skills we may consciously and no doubt rationally develop, such as walking and the playing of a musical instrument, but the actual actions are neither rational nor irrational. There are also preferences that may have no basis in rationality, for example, my preference for coffee in the morning and tea in the afternoon. (Preferences, can of course, be rational too – if I need to save money and I buy supermarket own-brand items because they are cheaper and do the job as well as the branded alternatives.)

There may well be other categories of actions that could not be classed as either rational or irrational.

So most people are not “irrational”, but rational actions must of necessity be a small (even though very significant) part of our lives. So I think it makes no sense to talk of people, or their behaviour, as being rational or irrational. The only meaningful use of the word ‘rational’ is in relation to specific arguments or actions where the aim or problem is well-defined enough to apply reasoning or relevant information to it.

I can see no validity in asserting that all human behaviour, or a defined part of it (as some economists do) is ‘rational’ in some way. You may assert that a particular conscious action or class of actions, by a particular person or all persons, is rational or irrational (if the latter is possible). But this is a hypothesis, subject to all the limitations of conjectural knowledge as Popper described. And, in general, you will never have the inside knowledge to adequately judge the hypothesis.

The chief executives of both Lehmann Brothers and the Royal Bank of Scotland seem to have had the aim of making their middling institutions into big ones of their kind, and they seem to have found the methods to do that.  Does that make them ‘rational’? Both were apparently warned, using sound arguments, that they were leading to likely disaster, and they ignored or disadvantaged the persons making the warnings. Does that therefore make the CEOs ‘irrational’? It seems to me that it is worthwhile to try to understand the processes that let up to the failures, but would trying to interpret the actions as ‘rational’ or ‘irrational’ add anything to our understanding? I doubt it.

Update 22 July 2009: Heresy Corner, writing about the UK government’s policy on the DNA database, illustrates how this is not merely a philosophical debate. The DNA database policy is founded on an ideological position that criminal behaviour must be a ‘rational’ decision. It also shows that ‘neoliberalism’ is not necessarily about individual freedom from government control.

“Deep Waters Trust” out of its depth

Darwin in Shrewsbury [BPSDB] You might think that if creationists want to criticise the theory of evolution by natural selection, as understood and used by the overwhelming majority of biologists, then they would try to understand the theory. Especially as the series of meetings in Shrewsbury, Darwin’s birthplace, was billed as “an assessment of the evidence for design which has emerged through the advances in science since publication of his [Darwin’s] On the Origin of Species in 1859”.

I have already commented on the fact that there didn’t appear to be any books explaining evolution on the bookstall at the Meyer meeting. Perhaps the organisers were afraid that if they understood evolution they may come to be convinced by it.

That the Shrewsbury Deep Waters Trust misunderstands the theory is clear from the press release put out before the meeting. Obviously, to understand the theory properly, and how it agrees with the evidence, you need to read a lot more than I can write here.

1. Because we are based in his birthplace we see a need for a different approach to Darwin from the extreme positions of devotion and hostility that are commonly adopted: while we don”t accept the conclusions of modern neo-Darwinians about evolution we respect Charles Darwin himself – both for his theory of Natural Selection and for his honesty in acknowledging the possibility that evolution might be proved false. There is evidence in his writings and those of his contemporaries that shows where he believed his theories were in need of confirmation by future research.

Scientists are not “devoted” to Darwin, although admittedly there appears to be a lot of hostility to him in some quarters. The commemoration honours a great scientist – but scientist is the word.  All scientists expect that their theories (if important) will be tested and questioned in the future. This questioning and testing is called “research”. In the case of Darwin’s theory – evolution by natural selection – the theory was vitally important and had implications over a wide range of science.  It could have been disproved by discoveries, not only in natural history and paleontology (the study of fossils) but also by discoveries in geology and astronomy and even physics. And, especially, by discoveries in the new science of genetics which was just beginning, unknown to Darwin, during his lifetime.

All this evidence has tested the theory to the maximum, and it has survived, improved, since Darwin’s day. It’s the only theory that matches the vast amount of evidence that has been collected. And it is still being tested.

2. Christians believe that God, not random mutations, is responsible for the design that underlies the world we live in. Particular recent evidence of design of which Darwin was not aware is in DNA – the genetic code: it is a language, containing information that controls the formation and operation of cells. It exists independently of the material from which the cells are made.

It may come as a surprise to the Deep Waters Trust that evolutionary scientists, even the most atheistic ones, do not believe that “random mutations” are responsible for design, or the appearance of design, in living organisms. Darwin’s insight was the selection of particular organisms by the environment – those that reproduce most successfully in the environment – enabling their genetic material to persist and become predominant. This is called “natural selection”. Darwin knew nothing about the genetic mechanism, of course, but the fact that it is consistent with evolution in all respects is one of the successes of Darwin’s theory.

3. Evolution involves progress “up” the evolutionary tree, each step requiring the addition of information to the genetic code.

No, it doesn’t! This is a serious misunderstanding. Evolution is about adaptation, not progress. The outcome of evolution, as it has happened, is that there are some complex organisms (a few of which think they rule the world), but simple organisms are just as evolved. Think of the bacteria – they live and exist much as the earliest of their kind, but they have evolved to live in all sorts of ecological niches.  There is possibly a larger mass of bacteria on the planet than of all other organisms put together. And loss of function is common in evolution – think of flightless birds.

Random processes do not produce meaningful information. Some Christians believe God used evolution to bring about his purposes, producing more complex designs progressively by stages. Others believe the DNA evidence is better interpreted as demonstrating a gradual loss of information as species change through Natural Selection. (Loss of genetic information produces greater variety in sub-species, but not the ability to change from a simpler species into a more complex one.) The ancestors of today’s species would have been fewer, more elaborate, forms containing all the genetic information from which present day life has descended.

Creationists abuse the idea of “information” and “complexity” (which seem to be interchangeable to them) by using the words in different ways so that you think they are talking about the same thing when, really, they are changing the meaning as they go along. In fact, there does not seem to be any generally accepted definition of either “information” and “complexity” that applies in understanding evolution. Dr Stephen Meyer, in his lecture, used the Shannon definition of information (as was clear from the slides he displayed). This definition specifically relates to understanding the transmission of information down a predefined communications channel. What relationship it has to evolution needs to be demonstrated. Dr Meyer’s use of it seemed to be mainly to give an apparently scientific appearance to a non-scientific argument.

4. The debate is sometimes portrayed in terms of a conflict between science and religion, where science suggests that life has evolved as a result of random processes, while religion claims God has brought it about deliberately.

Some believe both these views can be held at the same time; others that they are mutually exclusive, and that only with “blind” faith – faith despite evidence to the contrary – can one claim that both are true. We believe there is a third position that needs to be explored: the possibility that the scientific evidence is best interpreted as confirming design, not randomness.

No, as I said before, no scientific theory holds that life evolved purely as a result of random processes. That makes a difference.

To make ID (or creationism) scientific, what you have to do is show that it makes specific predictions about the evidence (fossils, genetic makeup of species, or whatever) that are different from (the real) theory of evolution by natural selection. And then show that the actual evidence agrees with ID (or creationism) and not with evolutionary theory. This is the challenge to the Discovery Institute or anyone else. And ID and creationism have always failed this challenge.

5. We believe there is not one debate but two: one debate about interpretations of the scientific evidence – what has come to be known as Intelligent Design versus random processes – and another between two faith positions: an originally good world which has been in decline as a consequence of human wrongdoing, or an originally simple and amoral world which has been evolving into something more complex and better.

These are both false dichotomies – a logical error. Are there really only two possible positions in each argument? I can think  of many positions in each case. I won’t bore you with mine, except to say that my position is emphatically not a “faith” position, as I would change it if any convincing contrary evidence was presented.

Stephen Meyer’s book

Dr Meyer’s book has now been published. I shall not be buying a copy, as his lecture suggested that there was nothing in it that is both significantly new and interesting. Other people, better qualified than I am to do so, will no doubt be reviewing it in time, and I shall look forward to reading their reviews.

Dr Meyer’s main theme in the lecture was a bit like the following argument. Suppose he had claimed that it was impossible to create a baby, because it is highly improbable that all the chemical components of a baby could come together in the right combinations.  We would surely argue that he is wrong, because babies are not made by the process of assembling all the chemicals at once. In fact, a baby is assembled slowly by processes in which the baby’s genes interact with his environment (including the mother’s womb and the outside environment). Similarly, the evolved biosphere is a product of continual processes of interaction between the organisms’ genetic materials and their environments.


The Deep Waters Trust seems to have gone into hibernation since the event. There is no sign of the recordings promised from the February event. The aims of the charity are given as “The advancement of education in the public arena of the relationship of belief in a creator god based on the Holy Bible and scientific discovery, philosophy, theory and investigation”.

See also

Creationism in Darwin’s birthplace

Not quite so honest to Darwin (or anyone else)

More about Stephen Meyer’s lecture on Intelligent Design

Other creationists who crashed the Darwin party

More about Stephen Meyer’s lecture on Intelligent Design

[BPSDB]Rather late, this is my comment on the lecture given in Shrewsbury (Darwin’s birthplace) in late February 2009 by Dr Stephen Meyer from the Discovery Institute. This was billed as reviewing “Darwin’s life and theories from a new perspective – an assessment of the evidence for design which has emerged through the advances in science since publication of his On the Origin of Species in 1859” . It was also advertised as Dr Meyer’s first public lecture on the theme of his new book, “Signature in the cell: DNA and the evidence for Intelligent Design”.

The book has not yet been published, so I can’t yet compare it with the lecture. However, it did seem to me that there was very little in the lecture that was actually new – I have been following the debate between biologists and ID proponents for some years now. It wasn’t possible for me to take notes fast enough to give a point-by-point analysis of the lecture.

The lecture was divided into two halves, with a short break of five minutes. The crucial argument was presented early in the second part. I’d almost call it a “climax”, because Dr Meyer is a showman: he knows how to ramp up the excitement and carry the audience with him. Of course, in this case the audience was largely sympathetic to his aim and uncritical towards his arguments.

There was, in reality, very little mention of evolution in the lecture – mostly quotations from Darwin and other scientists – although the whole thing was carried off so that I suspect that most in the audience really believed that they were witnessing a powerful critique of evolution.

The lecture began with a few comments about Darwin and evolution, there was some talk about “chemical evolution” and the work of Oparin and Miller, and then much of the first part was given over to explaining the basics – what seemed to me to be perfectly acceptable explanations of the shapes of DNA and proteins, and how the functions of proteins depend on their shapes. Then he went on to more contentious matters, including the ID proponents’ favoured idea of “specified complexity”.

The climax came after the break. Dr Meyer took the audience through the calculation of the probability of a protein (of 150 amino acids) assembling itself from its units – vanishingly small, of course. I could feel the excitement of the audience close to me as he approached the result and there was laughter as he calculated the final result. They must have felt that Dr Meyer had delivered the coup de grace to evolution by natural selection.

This was really the core of the lecture. The rest consisted largely of unsupported assertions of ID dogma, for example, that undirected natural processes do not produce large amounts of “specified complexity”, that creation of new information is habitually associated with conscious activity, that the functional integration of proteins, DNA and ATP is evidence for design, and that ID has been justified by predicting that so-called ‘junk DNA’ would be found to be functional (I’m not a biologist, but my understanding is that some noncoding DNA is known to have important functions, but that there is good evidence that much DNA has no function).

The lecture depended largely on what might be compared with a David Copperfield illusion: Dr Meyer did not demonstrate what the audience were no doubt expected to think he had demonstrated, and the whole effect depended on the audience seeing what it wanted to see and not questioning what they believed they had seen and heard.

The calculation of the improbability of a protein is irrelevant, because it entirely omits the very process that ID-ists claim to question – evolution by natural selection. The evolutionary process is what made it possible to observe the proteins we see today carrying out the functions that they do. No evolutionary biologist imagines that any functional protein somehow had to be assembled all at once out of its individual amino acids. Proteins evolved in organisms, through relatively small changes in the genetic material (changes that accorded with the laws of chemistry and physics), and were selected for or against by the environments prevailing at the time. No step required any probability that was unreasonably small in the available time frame.

What’s more, to look at the protein and its function the way Dr Meyer did is to get the whole thing backwards. The protein and its function were not “specified” first and the evolutionary process tailored to creating it. Both the protein and its function are as they are because they are the outcome of a whole series of changes that were possible at each step. They may not be the best possible way to carrry out the function, but they exist both because they are the result of a series of physically and chemically possible changes and they enable the organism to function in its current environment – specifically, to reproduce successfully. In many cases, it is now possible to trace the steps by which the current system and its function came about – see, for example, the debate between biologists and ID-ists over the biochemistry of blood clotting.

Not quite so honest to Darwin (or anyone else)

I attended the final session of the Shrewsbury “Honest to Darwin” series of events, which was a lecture by Stephen C Meyer of the Discovery Institute. I hope later to write more fully about the apparent purpose of these events and Dr Meyer’s lecture. In the meantime, here are a few comments.

This event was promoted by the organisers (the Shrewsbury Deep Waters Trust) as an “Intelligent Design event“, and to “look honestly at Darwin’s original views and their relationship to contemporary neo-Darwinism”. However, the person who introduced Dr Meyer and closed the whole programme (apparently Martin Charlesworth, Pastor of Barnabas Community Church, Shrewsbury), in his closing remarks, made it clear that the purpose of the week was against evolution and to promote theistic views and Christianity.

The lecture was attended by a capacity audience I estimated as approaching 200, and I got a sense that there were few sceptics there. The books on sale at the bookstall consisted of Christian and creationist books and some of the ‘answers to Dawkins’ genre. I did not see any titles devoted to explaining evolutionary theory! This is a shame, as the person who wrote the press release advertising the series of events clearly had no real idea of what evolution is about.

I missed the previous speaker, who was Andy McIntosh. Prof. McIntosh has in the past claimed that evolution is contrary to the second law of thermodynamics – this is not only false, but a real howler. Whether he made this claim to the gathering in Shrewsbury I do not know.

Dr Meyer’s lecture didn’t seem to contain much new (even though it was supposed to be based on his as-yet unpublished book) and heavily emphasised “specified complexity”. At the end of the lecture, one of the audience asked when the ideas in Dr Meyer’s lecture would reach “the schools”. Dr Meyer gave a glowing account of the research into Intelligent Design that the Discovery Institute claims is being carried out by many scientists, and how ID is supposedly gaining ground in science.

So I took the opportunity to ask what names I should look out for in order to follow this research. Dr Meyer named one name only: Douglas Axe, who was in the audience and is apparently the director of the Biologic Institute set up by the Discovery Institute itself. Perhaps the new book will contain an account of some of this research.

The unanswered questions of evolutionary theory are the sign of a live and vigorous research programme that brings together disparate parts of biology and geology, and is extending knowledge all the time. Intelligent Design is a backwater in which the same few people say the same few things over and over again. Even though the audience was uncritical and most of its members were probably desperate to believe Dr Meyer’s assertions, I still find it dishonest to try to represent ID to them as being some body of research comparable to mainstream biology, when in fact it is miniscule and primarily concerned with politics, not science. [bpsdb]

Creationism in Darwin’s birthplace

The creationists are in town this week in Darwin’s birthplace. I have been out of town myself so could not attend any of the weekday sessions, but I do hope to get to the Saturday lecture, which stars Stephen C. Meyer, one of the founders of the Discovery Insitute, the begetters of the “Intelligent Design” (ID) idea. If I can get there, I’ll report back to the local humanists and comment here.

Interestingly, although the Disco people like to promote ID as a “scientific” theory, without actually doing any science,  the inspiration behind the programme in Shrewsbury appears to be religious, and specifically Christian. The event information (not on line, as far as I can find) says: “We believe there is not one debate but two: one debate about interpretations of the scientific evidence – what has come to be known as Intelligent Design versus random processes – and another between two faith positions: an originally good world which has been in decline as a consequence of human wrongdoing, or an originally simple and amoral world which has been evolving into something more complex and better.” No room allowed here for any other interpretations, apparently, particularly the possibility that you can have a view on evolution that is not a “faith position”.

There is the common straw man version of evolution: “Evolution involves progress “up” the evolutionary tree, each step requiring the addition of information to the genetic code. Random processes do not produce meaningful information.”

If anyone has any information about the weekday presentations, I’d be interested to hear about them.  [bpsdb]

Word: Theory

(revised text)

A scientific theory is fundamentally an explanation of phenomena. These phenomena may be observed in the world around us, for example, the tides in the oceans or the different varieties of living things. They may be observed with the aid of devices made for the purpose, for example, observing distant galaxies with a telescope or bacteria through a microscope. The observations we want to explain may also be the result of highly contrived experiments, for example, the collisions of particles in a particle accelerator or chemical reactions in glassware.

In ordinary life, we may look for explanations of single events, such as “who left the door open?”, “how did that batch get contaminated?” or “how did that person die?” We may even talk of having a ‘theory’ about how something happened. Usually this means that we are not sure about the explanation. If we are sure, then we would usually describe the explanation as a fact.

Scientists use the word “theory” in a very different way. A scientific theory attempts to do two things as well as explain. It attempts to generalise: to cover all the occurrences of a particular phenomenon. In the theory of gravity, there are not separate explanations of each individual case of gravitational attraction, for example, falling objects on the earth’s surface. The theory describes what is common to all kinds of gravitational attraction, including objects falling on the moon, the tides, the motions of the planets and the structures of the galaxies. If you were looking at these different things without the benefit of a modern education you might not recognise that there was a single explanation linking these various phenomena. It took Newton’s genius to do that.

A good theory also unifies many different phenomena, where the connection may be very difficult to see. For example, the theory of plate tectonics became accepted during the 1960s because it explained not only old and otherwise inexplicable phenomena such as the existence of long mountain chains (like the Himalaya and Andes chains) that are mostly made up of tens of kilometres thickness of rocks formed from the erosion of older mountains, but also the new observations that were being made of  long volcanic ridges and very young crustal rocks under the oceans.

Observations are tests of theories. Most times when scientists build an apparatus to do an experiment, they have a theory in mind. If the observations that are made with an apparatus are not consistent with the theory, then of course the experimenter will check the design of the apparatus first, but if nothing can be found wrong with it, then the theory is refuted, and a new theory, or an improved version of the old theory, is required.

It is important to realise that, in science, the word ‘theory’ has no implication about how well understood a theory is, about how certain it is, or how many practising scientists in that field accept it*. A new theory must obviously go through a stage where most people regard it as provisional and not properly tested. This was the case with the theory of plate tectonics in the 1950s-1960s, although the theory had been around for a few decades before that. Nowadays, every practising geologist accepts plate tectonics, although there are always details that are in dispute. For example, we know that Iceland sits on a spreading ridge in the Atlantic, but there is currently disagreement over exactly what is happening under Iceland to make it such a prominent feature.

Similarly, there are no practising biologists, in fields related to evolution, who doubt that evolution by natural selection is correct, although there are always important details to be discovered and clarified. The important thing to remember is, that although we can never be 100% certain about a theory, there are many theories that we have no scientific reasons to doubt are fundamentally true. These include the theory of gravitation, which is used to make predictions of eclipses and other movements in the solar system accurate to fractions of a second. Well-established theories also include quantum theory and the theory of evolution by natural selection. The reason why these theories are accepted is that there is no other explanation in each case that fits the full range of obsrvations so well.

*Sometimes the word ‘hypothesis’ is used to describe a new theory that has not much observational support, but there is no consistency about this usage.