 Such Kinds are marked not by the possession of some metaphysical essence—but by their marking “an indeterminate multitude of properties not derivable from one another” (System, VII: 126). Some groups of objects share characteristics because those characteristics are the very grounds of their being grouped together—Real kinds share an unknown number of similarities because they have a shared nature. The notion is closely, though not unproblematically, related to the modern notion of a Natural Kind (Magnus 2015).