A second proposal offers a regularity account of constitutive relevance modeled on Mackie’s notion of understanding a cause as an INUS condition: an Insufficient but Non-redundant part of an Unnecessary but Sufficient condition for the effect in question (Mackie 1974; see also Cummins 1983). On this account, a constitutively relevant component is an insufficient but non-redundant part of an unnecessary but sufficient mechanism for a given phenomenon (Couch 2011; see also Harbecke 2010, 2014). Allow that any number of mechanisms might suffice to bring about S’s ψ-ing; each possible sufficient mechanism is then unnecessary for ψ-ing. Each of these mechanisms is made of components, none of which is alone sufficient to produce the behavior of the mechanism as a whole, but each of which is necessary in the context of the mechanism for S to ψ.  This account presupposes the idea of being necessary in context, and one might reasonably worry about sorting accidentally correlated Xs from Xs that in fact make a difference to S’s ψ-ing.