 Tom Doyle, Andreas Hüttemann, Ken Waters, Rob Wilson, and two anonymous referees provided useful suggestions on earlier versions of this entry. We appreciate the many further colleagues who gave us valuable feedback. Ingo Brigandt’s work on this essay was funded with an Izaak Walton Killam Memorial Postdoctoral Fellowship by the  Killam Trusts  of Canada and with Standard Research Grant 410-2008-0400 and Insight Grant 435-2016-0500 by the  Social Sciences and Humanities Research Council of Canada.  Alan Love’s work on revisions of this essay was supported in part by grants from the  John Templeton Foundation  (“Complexity, emergence and reductionism:  toward a multilevel integrative analysis of the brain and cognition,” ID 24426; “Integrating generic and genetic explanations of biological phenomena,” ID 46919).