Downward causes and appropriate abstractions: Alternative argument for Noble's position.

In his article of this issue, Denis Noble argues that causation can occur at and between any scales - rather than levels - in nature. I agree with this conclusion, but do not find his reasoning fully persuasive. I offer an independent but related argument for his position, which focuses on the role of the process of abstraction in identifying and understanding a phenomenon. My aim is to give the reader another way to think about causal talk within the framework of Systems Biology.