Pluto and the Platypus: An Odd Ball and an Odd Duck — On Classificatory Norms

Some astronomers 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 require that we exclude Pluto from the planets. But the obvious alternative, that classificatory revision is a matter of arbitrary choice, is also unpalatable. 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 argue that the “norm-ladenness” of classification has philosophically significant ramifications for how we think about scientific disputes and debates in the metaphysics of classification such as the monism/pluralism debate.