This paper examines three exemplary theories of living organization with respect to their common feature of defining life in terms of metabolic closure: autopoiesis, (M, R) systems, and chemoton theory. Metabolic closure is broadly understood to denote the property of organized chemical systems that each component necessary for the maintenance of the system is produced from within the system itself, except for an input of energy. It is argued that two of the theories considered—autopoiesis and (M, R) systems—participate in a hylomorphist pattern of thinking which separates the “form” of the living system from its “matter.” The analysis and critique of hylomorphism found in the work of the philosopher Gilbert Simondon is then applied to these two theories, and on the basis of this critique it is argued that the chemoton model offers a superior theory of minimal life which overcomes many of the problems associated with the other two. Throughout, the relationship between hylomorphism and the understanding of living things as machines is explored. The paper concludes by considering how hylomorphism as a background ontology for theories of life fundamentally influences the way life is defined.