In recent years new approaches to the integration of economics and thermodynamics have been developed which build on the physics of open non-equilibrium systems, the so-called ‘Maximum Entropy Production Principle’. I review these contributions in the light of the implications for economic ontology, i.e. the question what the fundamental constituents of real world economic phenomena are. I argue in favor of the ‘naturalization’ of economic ontology, using the phenomenon of economic growth as my workhorse, and I explore the implications for the cross-disciplinary foundations of ecological economics. The paper shows how economic growth can be conceived as a ‘natural’ process that is driven by fundamental physical forces. The argument proceeds in three steps. After a short review of recent research on the linkage between energy and growth, I establish the connection with bioeconomic theories about evolution that allow restating the role of Lotka's Maximum Power Principle (MPP) as a property of open non-equilibrium flow systems with sufficient degrees of freedom of structural adaptation. The MPP is then related to the recent literature on Maximum Entropy Production (MEP), especially as deployed in the Earth Sciences. Economic growth can be seen as resulting from evolutionary adaptations of flow gradients in economic systems that increase throughputs of exergy and generation of work, and which thereby enhance the capacity of the Earth System to maximize entropy production. This framework offers fresh perspectives on a number of issues in research and policy, which I discuss in the conclusion.