  The purpose of this paper is to discuss the various types of physical
universe which could exist according to modern mathematical physics. The paper
begins with an introduction that approaches the question from the viewpoint of
ontic structural realism. Section 2 takes the case of the 'multiverse' of
spatially homogeneous universes, and analyses the famous Collins-Hawking
argument, which purports to show that our own universe is a very special member
of this collection. Section 3 considers the multiverse of all solutions to the
Einstein field equations, and continues the discussion of whether the notions
of special and typical can be defined within such a collection.
