30 | NewScientist | 23 April 2011

One of the most remarkable features of science is the contrast between the enormous power of its explanations and the parochial means by which we create them. no human has ever visited a star, yet we look at dots in the sky and know they are distant white-hot nuclear furnaces. Physically, that experience consists of nothing more than brains responding to electrical impulses from our eyes – which can detect light only when it is inside them. That it was emitted far away and long ago are not things we experience. We know them only from theory.

How do we know? And how do we know about conditions just after the big bang? Or about the future, which we cannot measure at all? How do we know that a new microchip will work, or that a new vaccine will prevent a particular disease, before they even exist?

evidently we do know: look at the remarkable changes to our society that have come from science. Progress rapid enough to be noticed, and that has continued over many generations, has been achieved only once in the history of our species. It began at roughly the time of the scientific revolution, a period that included improvements in technology, political institutions, moral values, art and every aspect of human welfare.

We call this the enlightenment, a term that historians use to denote a variety of trends, some of them violently conflicting, but all of them rebellions against authority. But mere rebellion – a common event in history – cannot explain how science provided a stream of ever truer explanations. In my new book, The Beginning of Infinity, I offer a new answer to this question.

For thousands of generations, we were in the dark. Our ancestors gazed at the night sky wondering what stars are – which was exactly the right thing to wonder about – using eyes

What is the secret of science’s success in understanding our world? It’s to do with the quality of its explanations – though there is a twist in the tale, says David Deutsch

The source of all progress

ProfileThis essay is based on David Deutsch’s latest book, The Beginning of Infinity (Allen Lane). Deutsch is a member of the Centre for Quantum Computation, at the Clarendon Laboratory, University of Oxford

and brains anatomically indistinguishable from ours. In every other field too, they tried to observe the world and to understand it. Occasionally they recognised simple patterns in nature, but when they tried to discover the underlying features of reality they failed almost completely. At the time of the enlightenment they mistakenly believed that we “derive” knowledge of these features from the “evidence of our senses”, or “read” it from the “Book of nature” by making observations, the doctrine called empiricism.

But science needs more than empiricism. Consider an audience watching a conjuring trick. The problem is not to predict that the trick will appear: I can predict that whenever a conjurer shows me an empty hat, something will later emerge from it. The problem is how the trick works, and that requires an explanation – a description of the unseen reality that accounts for the appearance.

For a new explanation one in turn needs creativity. To interpret dots in the sky as white-hot, million-kilometre spheres, one must first have conjured up the idea. That happens through guesswork – but guesswork usually produces errors, which is why observation is indeed essential to science, though not in the way supposed by empiricism. Its main use is to choose between theories that have already been guessed by rearranging, combining, altering and adding to existing ideas.

Sustained progress through alternating guesswork and criticism requires a tradition of criticism. Before the enlightenment, that

was a very rare sort of tradition. Usually, the whole point of traditions was to maintain the status quo and to defer to authority.

In turn, the scientific tradition of criticism soon led to the rule that theories must make testable predictions. This alone cannot be the secret of the success of science, however, because testable predictions have always been common too. every prophet who claims that the sun will go out next Tuesday has a testable theory. Science must consist of testable explanations.

Though they helped to spur the enlightenment, testable explanations are also not in themselves sufficient, since they existed long before science. Consider the ancient Greek myth explaining the seasons: Hades, god of

OPINION THE BIG IDEA

110423_Op_Big_Idea.indd 30 15/4/11 10:46:51

23 April 2011 | NewScientist | 31

DEN

IS/R

EA

the underworld, kidnapped Persephone, goddess of spring, and later released her in return for marrying him. While in the underworld, she had to eat six pomegranate seeds, which compelled her to return every year. Whenever she does so her mother, Demeter, goddess of the harvest, becomes sad and cools the world so that nothing can grow.

That myth, though false, is an explanation. It is also testable: if winter is caused by Demeter’s sadness, then it must happen simultaneously everywhere on Earth. Now, if the ancient Greeks had known that summer happens in Australia at the very moment when Demeter is at her saddest, they could have inferred that their explanation was false.

The problem is that they could easily vary

this explanation: perhaps Persephone was not released, she escaped. Every spring, she takes revenge on Hades by cooling his realm with her spring air, which displaces hot air into the northern hemisphere. Every winter, sad Demeter banishes warmth from her vicinity, sending it south. Because their underlying explanations are easily varied, myths can accommodate any new experience. Thus the testability of the Persephone theory alone was no help in making progress – and indeed, none was made.

Contrast this with the true explanation of seasons: that the Earth’s axis is tilted relative

to the plane of its orbit. Hence, during half of each year, the northern hemisphere is tilted towards the sun while the southern hemisphere is tilted away, and vice versa during the other half. Whenever the sun’s rays fall vertically in one hemisphere (thus providing more heat per unit area of the surface), they are falling obliquely in the other.

That is a good explanation – hard to vary because all its details play functional roles. The axis-tilt theory also predicts that the seasons will be out of phase in the two hemispheres. Had that not been observed, the theory would have been refuted just as, in the event, the myth was refuted by the fact it was observed. But the difference is this: if the axis-tilt theory had been refuted, no variant could make tilted axes unify the

seasons across the planet. Fundamentally new ideas would have been needed.

Let me clarify one point about hard-to-vary theories. Good explanations are often strikingly simple, and sometimes a bad explanation can be improved by deleting superfluous features. This has given rise to a misconception, sometimes called Occam’s razor, that one should always seek the “simplest” explanation, with the minimum assumptions. However, there are plenty of simple ideas that are nevertheless easily variable (such as “Demeter did it”). And while unnecessary assumptions make an explanation bad by definition, there have been many mistaken ideas of what is “necessary” in an explanation (For instance, instrumentalism considers explanation itself unnecessary, as do many other bad philosophies.) So I would argue that science is the quest for hard-to-vary explanations.

There’s a caveat, though. Solutions always reveal new problems. So one must also always seek a better hard-to-vary explanation. That, at its heart, is the scientific method. As Richard Feynman remarked: “Science is what we have learned about how to keep from fooling ourselves.” Because it is prior to experimental testing, the practice of requiring good explanations can drive objective progress even in non-scientific fields. This is exactly what happened in the Enlightenment. Although the pioneers of that era did not put it that way, it was, and remains, the spirit of the age. It is the source of all progress. n

“ The practice of requiring good explanations… is the source of all progress”

Staring at the stars may be the right thing to do, but science needs more than empiricism

For more opinion articles and to add your comments, visit newscientist.com/opinion

110423_Op_Big_Idea.indd 31 15/4/11 10:47:02