Introduction

We know that nature is governed by mathematics and symmetries. Not very long ago, it was an article of faith among most physicists that everything about physics would eventually be explained in terms of fundamental symmetries — that nothing in the make-up of physical laws is accidental, that nature ultimately has no choices, and that all the properties of particles and fields are fixed purely by mathematics.

In the thirty years since modern anthropic reasoning was introduced into cosmology [1, 2], the competing idea that anthropic selection might have an indispensable role in fundamental physical theory has gradually become, if not universally accepted, at least mainstream. There are now concrete physical models for realizing anthropic selection in nature. Cosmology has provided not only a concrete mechanism (inflation) for manufacturing multiple universes, but also a new phenomenon (dark energy) whose value is most often explained by invoking anthropic explanations. String theory has uncovered a framework by which many different symmetries and parameters for fields can be realized in the low-energy, 4-dimensional universe; this depends on the topology and size of the manifold of the other seven (truly fundamental) dimensions and on the configurations of p-branes within it, especially the local environment of the 3-brane on which our own Standard Model fields live.