Science of Discworld III

is blind.
    The scientific vision of the planet that is currently our only home, and of the creatures with which we share it and the universe around it, has attained its present form over thousands of years. Thedevelopment of science is mostly an incremental process, a lake of understanding filled by the constant accumulation of innumerable tiny raindrops. Like the water in a lake, the pool of understanding can also evaporate again – for what we think we understand today can be exposed as nonsense tomorrow, just as what we thought we understood yesterday is exposed as nonsense today. We use the word ‘understanding’ rather than, ‘knowledge’ because science is both more than, and less than a collection of immutable facts. It is more, in that it encompasses organising principles that explain what we like to think of as facts: the strange paths of the planets in the sky make perfect sense once you understand that planets are moved by gravitational forces, and that these forces obey mathematical rules. It is less, because what may look like a fact today may turn out tomorrow to have been a misinterpretation of something else. On Discworld, where obvious things tend to be true, a tiny and insignificant Sun does indeed revolve round the grand, important world of people. We used to think our world was like that too: for centuries, it was a ‘fact’, and an obvious one, that the Sun revolved round the Earth.
    The big organising principles of science are theories , coherent systems of thought that explain huge numbers of otherwise isolated facts, which have survived strenuous testing deliberately designed to break them if they do not accord with reality. They have not been merely accepted as some act of scientific faith: instead, people have tried to falsify them – to prove them wrong – but have so far failed. These failures do not prove that the theory is true , because there are always new sources of potential discord. Isaac Newton’s theory of gravitation, in conjunction with his laws of motion, was – and still is – good enough to explain the movements of the planets, asteroids and other bodies of the solar system in intricate detail, with high accuracy. But in some contexts, such as black holes, it has now been replaced by Albert Einstein’s theory of general relativity.
    Wait a few decades, and something else will surely replace that. There are plenty of signs that all is not well at the frontiers of physics.When cosmologists have to postulate bizarre ‘dark matter’ to explain why galaxies don’t obey the known laws of gravity, and then throw in even weirder ‘dark energy’ to explain why galaxies are moving apart at an increasing rate, and when the independent evidence for these two powers of darkness is pretty much non-existent, you can smell the coming paradigm shift.
    Most science is incremental, but some is more radical. Newton’s theory was one of the great breakthroughs of science – not a shower of rain disturbing the surface of the lake, but an intellectual storm that unleashed a raging torrent. Darwin’s Watch is about another intellectual storm: the theory of evolution. Darwin did for biology what Newton had done for physics, but in a very different way. Newton developed mathematical equations that let physicists calculate numbers and test them to many decimal places; it was a quantitative theory. Darwin’s idea is expressed in words, not equations, and it describes a qualitative process, not numbers. Despite that, its influence has been at least as great as Newton’s, possibly even greater. Darwin’s torrent still rages today.
    Evolution, then, is a theory, one of the most influential, far-reaching and important theories ever devised. In this context, it’s worth pointing out that the word ‘theory’ is often used in a quite different sense, to mean an idea that is proposed in order to be tested. Strictly speaking, the word that should be used here is ‘hypothesis’, but that’s such a fussy, pedantic-sounding word that people tend to avoid it. Even scientists, who should know better. ‘I have a theory,’ they say. No, you have a hypothesis. It will take years, possibly centuries, of stringent tests, to turn it into a theory.
    The theory of evolution was once a hypothesis. Now it is a theory. Detractors seize on the word and forget its dual use. ‘ Only a theory,’ they say dismissively. But a true theory cannot be so easily dismissed, because it has survived so much