Levels, nests and branches: Compositional organization and downward causation in biology

The idea of compositional levels of organization is deeply rooted into contemporary biology and its philosophy. I argue that when we take a closer look at the structure of biological systems, it turns out that hierarchical organization in nature is more complex and messy than most accounts of levels assume. More specifically, biological components tend to be very heterogeneous, which results in branching and tangled hierarchies instead of neat levels. Consequently, the traditional idea of levels should be treated as a heuristic abstraction that is only useful in some specific biological contexts. I demonstrate this approach by applying it to the debate on downward causation, where assumptions about levels play an important role and should be made explicit