Philosophy of science has a complicated relationship with metaphysics. Studying topics such as the nature of causation, laws of nature and space-time, it clearly engages in activities that merit classi cation as metaphysics. Yet the academic discipline itself was born in opposition to metaphysics. The positi vists were united in a shared distrust of metaphysics. Their suspicion ran so deep as to motivate a search for a demarcation between science and non- science, and science and speculative metaphysics in particular. Even today, philosophy of science appears caught in what Einstein (1933) called the '‘eternal antithesis between the two inseparable components of our knowledge -– the empirical and the rational'’ (p. 271). It wants to employ metaphysical speculation, but impressed with the methods of the subject it studies, it fears overreaching. Philosophy of science thus tries to walk a  ne line between scienti cally grounded meta physics and its more speculative cousins. Here I will try to dra some of the contour of this boundary, along the way introducing the reader to some of the relevant issues. Doing so is critical today, for we are in the midst of a major collision between two very large forces in philosophy that has a signi cant bearing on metaphysics. Whereas metaphysics and science were once one and the same  eld, natural philosophy, today there is a worrisome divide between the two. This separation is no doubt due to developments within both science and metaphysics. Physics, for instance, in part due to its distribution of incentives since World War II, is far less '‘philosophical'’ than it used to be (Holton 1986). Nineteenth-century physicists debated the reality of the electric  eld, but today few physicists debate the updated counterparts of this question for gauge  elds. The same goes for the measurement problem in quantum mechanics. Sometimes dubbed the '‘reality problem'’, the issue is really about the proper ontology suited to quantum theory, and it'’s hard to imagine a question of comparable importance in previous times being shunted aside as it o en is today...