 An interesting feature of this extended criticism of dichotomous, arbitrary division and the development of a systematic, multi-differentiae alternative is that it takes for granted the ‘naturalness’ of certain kinds that ‘the many’ have identified. As he says above, ‘these [birds and fish] have been defined by many differences’; earlier he had said that the terms ‘bird’ and ‘fish’ name a ‘similarity’, a group of animals that are ‘alike in kind’ that dichotomous division inappropriately tears apart (642b13–19). During the discussion of division there is very little clarification of what underlies this notion of similarity or likeness in kind. However, armed with a method of division suited to the study of animals, the discussion quickly turns to the question of what it is that makes it appropriate to treat all birds or all fish as a single kind, and to the specification of rules for identifying other such kinds. What ‘bird’ and ‘fish’ name are, he claims, kinds with “common natures, and forms not very far apart” (644b2–3). Prior to understanding these kinds through a scientific account of this common nature, we can—as the Posterior Analytics argues—grasp these kinds, i.e. grasp that all birds have a common  nature.[8]  We do this by noting that they have many general differences that vary in measurable ways—by the more and less, as he puts it.