Downward Causation and Levels

This chapter discusses a number of different notions of level, including levels as involving compositional relations and levels as relatively explanatorily autonomous. The notion of downward causation, understood as causation from upper to lower levels, is defended as legitimate. Downward causation is elucidated in terms of a relation called conditional causal independence: X is causally independent of Y conditional on Z when X is causally relevant (in the standard interventionist sense) to Z, Y is causally relevant to Z but conditional on the values of Z, changes in the value of X make no further difference to Y. When Y is a lower-level realizer of X and conditional independence holds, we can use Y rather than X to explain Z and the Y to Z relation is conditionally autonomous from X-level facts