Steel (2008) appeals to a somewhat weaker form of modularity in his probabilistic analysis of mechanisms—one that follows directly from Simon’s (1996 [1962]) idea of nearly decomposable systems.  On Simon’s view, the parts of a mechanism have more and stronger causal relations with other components in the mechanism than they do with items outside the mechanism. This gives mechanisms (and parts of mechanisms) a kind of “independence” or “objecthood” defined ultimately in terms of the intensity of interaction among components. Grush (2003), following Haugeland (1998), develops an idea of modularity in terms of the bandwidth of interaction, where modules are high-bandwidth in their internal interactions and low-bandwidth in their external interactions. On this view, modularity is not an all-or-none proposition but a matter of degree; mechanisms are only nearly decomposable. Craver (2007) argues that such a generic notion fails to account for the relevance of different causal interactions for different mechanistic decompositions; what counts as a part of a mechanism can only be defined relative to some prior decision about what one takes the mechanism to be doing. For criticisms of modularity, see Mitchell (2005) and Cartwright (2001, 2002).