ABSTRACT
One can explain the behavior of the whole in terms of the organized behaviors of its parts, and one can explain the behaviors of the parts in terms of the organized behaviors of their parts. Levels, on this view, are simply embedded mechanistic explanations. This chapter explicates this mechanistic view of levels, contrasts it with other senses of "level", namely aggregates, size, causal, and Oppenheim and Putnam's levels, and sketches its implications for emergence and reduction, for the ontological status of higher-level phenomena, and for thinking about the lowest level(s) in such hierarchies. The notion of mechanistic levels provides a compelling alternative to the Oppenheim and Putnam model. Like "level", "reduction" is used many ways. Sometimes, it expresses a thesis about explanation, about how one theory or law is explained by other theories or laws. Other times, it is a thesis about scientific integration and unity, about the relationships among the diverse branches of science.
