STEVEN FRENCH and JAMES LADYMAN REMODELLING STRUCTURAL REALISM: QUANTUM PHYSICS AND THE METAPHYSICS OF STRUCTURE ABSTRACT. We outline Ladyman’s ‘metaphysical’ or ‘ontic’ form of structural realism and defend it against various objections. Cao, in particular, has questioned the view of ontology presupposed by this approach and we argue that by reconceptualising objects in structural terms it offers the best hope for the realist in the context of modern physics. 1. STRUCTURAL REALISM – THE EPISTEMIC FORM Accommodating ontological change in science is a fundamental and long- standing issue in the philosophy of science. The fact that there has been ontological discontinuity across theory-change forms the basis of the so-called ‘pessimistic meta-induction’ which lists occurrences of such changes in the history of science and concludes, inductively, that it is very likely that the ontology of our currently accepted theories will also undergo radical revision (Laudan 1981). This is then presented as undermining sci- entific realism. However, we need to be careful about the form of realism the argument supposedly affects. Presumably it has to be a form of realism which insists that we can ‘read off’ ontology from current science – and in particular, physics – and thus come to some conclusion as to how the world is. An important issue then – to which we shall return – is how this term ‘ontology’ should be understood. There is a general type of response to the above concerns that has recurred through the history of the philosophy of science and which is ba- sically structuralist in nature. 1 It constitutes the heart of Worrall’s attempt to defuse the pessimistic meta-induction by insisting that, with regard to these shifts in ontology, ‘[t]here was continuity or accumulation in the shift, but the continuity is one of form or structure, not of content’ (Worrall 1996, 157). 2 This forms the basis of his epistemic version of ‘structural realism’ (SR) which, in our view, has reinvigorated the realist-antirealist debate. It is ‘epistemic’ because the central claim is that all that we know is this ‘form or structure’, whereas the ontological content, although retained, is unknowable. Worrall’s remark raises two fundamental questions: Synthese 136: 31–56, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.