 These problems are not just due to the difficulties of defining what an organism is. Similar issues arise when we consider the components of cells, such as the cell membrane and mitochondria—their subcomponents (e.g, lipid molecules and the outer mitochondrial membrane respectively) also exhibit too much heterogeneity to form a neat “level” (cf. Eronen 2013). Moreover, the same kinds of things can be parts in very different higher-level wholes: For example, hydrogen ions can be free-standing components in the oxidative phosphorylation mechanism that involves the cell membrane as a component, but they can also be components of the lipid molecules that make up the cell membranes (Bechtel 2008: 147).