Metaphysical realism is a view about the nature of reality that derives from science. It presupposes scientific realism, and seeks to construct a theory about the nature of reality based on this assumption. A principle that guides much of metaphysical realism is that first elaborated by John Fox (1987) in his ‘Truth-maker’ theory. It is the claim that what is true depends ontologically on what there is, i.e. that truth supervenes on being. Consequently, the task of constructing a metaphysics for scientific realism is seen as being that of providing an adequate set of truthmakers for the established truths of science and mathematics. It must begin with the science, draw conclusions about what truths need to be explained ontologically, and end with the required explanations.