 One attempt to provide a principled and practical way to characterize biodiversity comes from Maclaurin &amp; Sterelny (2008). James Maclaurin and Kim Sterelny start with the suggestion that we find the relevant “natural” units and then provide means for measuring variation with regard to those units. By ‘natural’ units, they mean ones that identify properties that are recognizable and are causally salient (Maclaurin &amp; Sterelny 2008: 10). As discussions of phenetic taxonomy have shown, measures of “overall similarity” are fraught with subjectivity. This subjectivity depends on a choice of distance metric and on how one articulates character traits (Ereshefsky 2000: 60–66). Contemporary taxonomists group taxa according to patterns of evolutionary descent, which is a method called “cladistics”, precisely to avoid such subjectivity given that the evolutionary history of life is natural in the above sense (Baum &amp; Smith 2013).