Pluto and the platypus: An odd ball and an odd duck - On classificatory norms

•The dispute about whether we discovered that Pluto is a planet is discussed.•This dispute is compared to the controversy about the status of the platypus.•Classificatory norms play a significant role in disagreement about classification.•Classificatory norms have important implications for controversies about pluralism.

Many astronomers seem to believe that we have discovered that Pluto is not a planet. I contest this assessment. Recent discoveries of trans-Neptunian Pluto-sized objects do not militate for Pluto's expulsion from the planets unless we have prior reason for not simply counting these newly-discovered objects among the planets. I argue that this classificatory controversy - which I compare to the controversy about the classification of the platypus - illustrates how our classificatory practices are laden with normative commitments of a distinctive kind. I conclude with a discussion of the relevance of such “norm-ladenness” to other controversies in the metaphysics of classification, such as the monism/pluralism debate.