 The proposal would be to define a phenomenon a as reductively explained by a phenomenon b iff a is mechanistically explained by b. Mechanistic dependence might be distinguished from a mereological relation, if mechanisms are not the sorts of things that have parts in the ordinary sense of the term as some have claimed (Craver 2005). On one reading, the entity explained in a mechanistic explanation is an event. A mechanistic explanation explains an event in terms of events that together form the event to be explained. On this view, mechanistic dependence is analogous to mereological dependence but not the same, and might thus complement the mereological approach to yield a fully-fledged characterization of reduction.