I suppose one could imagine laws of physics which would dictate that a world be exactly so, and not otherwise, allowing no detail to be varied. But what could dictate that those laws of physics be ‘the’ laws of physics? By considering a spectrum of possible laws, one could again consider a spectrum of possible worlds.

In fact the laws of physics of our actual world, as presently understood, have no such dictatorial character. So that even with the laws given, a spectrum of different worlds is possible. There are two kinds of freedom. Although the laws say something about how a given state of the world may develop, they say nothing (or anyway very little) about in what state the world should start. So, to begin with, we have freedom as regards ‘initial conditions’. To go on with, the future that can evolve from a given present is not uniquely determined, according to contemporary orthodoxy. The laws list various possibilities, and attach to them various probabilities.

The relation between the set of possibilities and the unique actuality which emerges is quite peculiar in modern ‘quantum theory’ – the contemporary all-embracing basic physical theory. The absence of determinism, the probabilistic nature of the assertions of the theory, is already a little peculiar… at least in the light of pre-twentieth-century ‘classical’ physics. But after all everyday life, if not classical physics, prepares us very well for the idea that not everything is predictable, that chance is important.