The Science of Discworld Revised Edition

while continuing to work well in their respective domains. With everything reduced to one Ultimate Rule, reductionism will have reached the end of its quest, and the universe will be completely explained.
    The extreme version of the alternative view is that there are
no
ultimate rules, indeed that there are no totally accurate rules either. What we call laws of nature are human approximations to regularities that crop up in certain specialized regions of the universe – chemical molecules, galaxy dynamics, whatever. There is no reason why our formulations of regularities in molecules and regularities in galaxies should be part of some deeper set of regularities that explains both, any more than chess and soccer should somehow be aspects of the same greater game. The universe could perfectly well be patterned on all levels, without there being an ultimate pattern from which all the others must logically follow. In this view, each set of rules is accompanied by a statement of which areas it can safely be used to describe – ‘use these rules for molecules with fewer than a hundred atoms’ or ‘this rule works for galaxies provided you don’t ask about the stars that make them up’. Many such rules are contextual rather than reductionist: they explain why things work the way they do in terms of what is
outside
them.
    Evolution, especially before it was interpreted through the eyes of DNA, is one of the clearest examples of this style of reasoning. Animals evolve because of the environment in which they live, including other animals. A curious feature of this viewpoint is that to a great extent the system builds its own rules, as well as obeying them. It is rather like a game of chess played with tiles that can be used to build new bits of board, upon which new kinds of chess piece can move in new ways.
    Could the entire universe sometimes build its own rules as it proceeds? We’ve suggested as much a couple of times: here’s a sense in which it might happen. It’s hard to see how rules for matter could meaningfully ‘exist’ when there is no matter, only radiation – as there was at an early stage of the Big Bang. Fundamentalists would maintain that the rules for matter were always implicit in the Theory of Everything, and
became
explicit when matter appeared. We wonder whether the same ‘phase transition’ that created matter might
also
have created its rules. Physics might not be like that, but biology surely is. Before organisms appeared, there couldn’t have been any rules for evolution.
    For a more homely example, think of a stone rolling down a bumpy hillside, skidding on a clump of grass, bouncing wildly off bigger rocks, splashing through muddy puddles, and eventually coming to rest against the trunk of a tree. If fundamentalist reductionism is right, then every aspect of the stone’s movement, right down to how the blades of grass get crushed, what pattern the mud makes when it splatters, and why the tree is growing where it is anyway, are consequences of one set of rules, that Theory of Everything. The stone ‘knows’ how to roll, skid, bounce, splash, and stop
because
the Theory of Everything tells it what to do. More than that: because the Theory of Everything is
true
, the stone
itself
is tracking through the logical consequences of those rules as it skitters down the hillside. In principle you could predict that the stone would hit that particular tree, just by working out necessary consequences of the Theory of Everything.
    The picture of causality that this viewpoint evokes is one in which the only reasons for things to happen are because the Theory of Everything says so. The alternative is that the universe is doing whatever the universe does, and the stone is in a sense
exploring
the consequences of what the universe does. It doesn’t ‘know’ that it will skid on grass until it hits some grass and finds itself skidding. It doesn’t ‘know’ how to splash mud all over the place, but when it hits the puddle, that’s what happens. And so on. Then we humans come along and look at what the stone does, and start finding patterns. ‘Yes, the reason it skids is because friction works like
this
…’ ‘And the laws of fluid dynamics tell us that the mud must scatter like
that
…’
    We know that these human-level rules are approximate descriptions, because that’s why we invented them. Mud is lumpy, but the rules of fluid dynamics don’t take account of lumps. Friction is