This paper addresses the question ‘what is an organism?’. Extant theories of organismality only provide a partial answer because they do not include an account of composition on which an ontology of living entities can be based. Here we develop a new account of what organisms are, based on a naturalistic answer to the special composition question, the bound state view. We argue that physical structure, including the existence of a boundary, is essential for life, and that, therefore, organisms are a particular kind of composite physical object – living objects. The bound state account of composition explains how composite physical objects exist in the world, and the property ‘life’ distinguishes the subset of those objects which are organisms. Our view obviates the need for disjunctive accounts of composition for living and non-living entities, placing ‘organism’ within the context of a broader scientific ontology, while at the same time providing a clear criterion of organismality that can be used in adjudicating problematic cases of biological individuality
