In my target article (Anderson this collection), I argued that the complexity of the function-structure relationships that give rise to direction selectivity in Direction-Selective Ganglion Cells Direction-Selective Ganglion Cells (DSGCs) and in the dendrites of Starburst Amacrine Cells Starburst Amacrine Cells (SACs) represent a challenge to componential mechanism as currently formulated (Craver 2008). First, I argued that distinguishing between the system S that exhibits the target phenomenon ψ, and the mechanism M in virtue of which it ψs allows one to paint a more nuanced picture of the various ways entities can be organized so as to give rise to observed function. Second, I suggested that the function-structure relationships in these particular cases appeared to violate the bottom-up hierarchical assumptions at the center of the componential mechanistic framework, which requires that the components of M in virtue of which a system exhibits ψ are at a lower level of organization than S. In the cases under discussion, I argued that some parts of the mechanism in virtue of which SAC dendrites function are at a higher level of organization than the dendrite, and that parts of the mechanism in virtue of which DSGCs function are at the same level. Moreover, I noted that in neither of these cases were all the entities that constituted M constitutive parts (components) of S.
To accommodate such cases, I recommended extending the notion of mechanistic constitution with the notion of an enabling constraint: mechanisms, we should say, enable function in systems by changing the relative probabilities of functional outcomes of activity in S. I suggested that this change would allow us to more accurately characterize the variety of structure–function relationships in the brain (and in other complex systems). However, in his commentary on my article (Kohler this collection), Axel Kohler argues that such an extension is unnecessary, for in fact the componential mechanistic framework of Craver and Bechtel (Craver 2008; Craver & Bechtel 2007) can accommodate these cases.
Kohler is correct. The extension is strictly speaking unnecessary, and componential mechanistic explanation can offer one plausible characterization of function-structure relationships in these cases. In fact, it is probably the case that no example or set of examples ever forces one to give up on an explanatory framework (certainly not one as well-motivated and useful as componential mechanism). What examples such as these can do, however, is illuminate the potential advantages of a new approach, and I would like to use the opportunity offered by this reply to reiterate what I take some of those advantages to be.