Seen from outside, as it sits in Rincewind's office, the entire human universe is a small sphere.

Large quantities of magic went into its manufacture and, paradoxically, into maintaining its most interesting feature. Which is this: Roundworld is the only place on Discworld where magic does not work. A strong magical field protects it from the thaumic energies that surge around it. Inside Roundworld, things don't happen because people want them to or because they make a good narrative: they happen because the rules of the universe, the so-called 'laws of nature', make them happen.

At least, that was a reasonable way to describe things ... until human beings evolved. At that point, something very strange happened to Roundworld. It began, in various ways, to resemble Discworld. The apes acquired minds, and their minds started to interfere with the normal running of the universe. Things started to happen because human minds wanted them to. Suddenly the laws of nature, which up to that point had been blind, mindless rules, were infused with purpose and intention. Things started to happen for a reason, and among these things that happened was reasoning itself. Yet this dramatic change took place without the slightest violation of the same rules that had, up to that point, made the universe a place without purpose. Which, on the level of the rules, it still is.

This seems like a paradox. The main content of our scientific commentary, interleaved between successive episodes of a Discworld story, will be to resolve that paradox: how did Mind (capital

'M' for 'metaphysical') come into being on this planet? How did a Mindless universe make up its own Mind? How can we reconcile human free will (or its semblance) with the inevitability of natural law? What is the relation between the 'inner world' of the mind and the allegedly objective 'outer world' of physical reality?

The philosopher Rene Descartes argued that the mind must be built from some special kind of material -mind-stuff that was different from ordinary matter, indeed undetectable using ordinary matter.