Chapter 4 brings the relationship between organism and habitat into sharper focus by tracing what oxygen does in a living body. It argues that the activity of oxygen requires that we reject a substantive understanding of bodies and that instead we conceive of living bodies as “energy in transition.” Elaborating this claim by examining the process of cellular respiration, the chapter notes that while every biochemical step in a cellular process is a critical condition of the next, the presence of oxygen in a cell is an index of the whole organism’s exposure to and utter dependence on its engagement with its habitat. The chapter draws on this insight to argue that we can only appreciate the micro-level biochemical processes that constitute living if we remember that they occur in whole organisms who engage with and are dependent on their habitats.