From a mechanistic perspective, levels are not monolithic divides in the furniture of the universe (as represented by Oppenheim and Putnam 1958), nor are they fundamentally a matter of size or the exclusivity of causal interactions within a level (Wimsatt 1976). Rather, levels of mechanisms are defined locally within a multilevel mechanism: one item is at a lower level of mechanisms than another when the first item is a part of the second and when the first item is organized (spatially, temporally, and actively) with the other components such that together they realize the second item. Thus, the mechanism of spatial memory has multiple levels, some of which include organs such as the hippocampus generating a spatial map, some of which involve the cellular interactions that underlie map generation, and some of which involve the molecular mechanisms that underlie those cellular interactions (Craver 2007). For more on levels, see Section 4.2 below.