 As natural as it is to interpret Locke as a natural kind realist, there are problematic passages for that interpretation. With respect to the similarities among real essences, which was the main concern of Leibniz and Stillingfleet, in IV.vi.4 Locke argues that the assumption that species are determined by real essences fails to tell us what would count as a member of any species. That is, because we have no knowledge of real essences, no general claims about species would be justified and no individual would obviously belong to a particular species. So the assumption that Leibniz and Stillingfleet propose appears to be theoretically unmotivated. Moreover, as Locke’s argument in III.vi.39 suggests, similarities in structure of the real essences, even if known, would still require us to decide which similarities (not to mention what degrees of similarity) would amount to similar enough to belong together in the same class: