Abstract. Stars are remarkably absent from reflections on natural kinds and classifications, with gold, tiger, jade, and water getting all the philosophical attention. It is a pity, for interesting philosophical lessons can be drawn from stellar taxonomy as regards two central debates about natural kinds, to wit, the monism/pluralism debate and the realism/antirealism debate. I show in particular that stellar kinds will not please the essentialist monist, nor will it please the pluralist embracing promiscuous realism a la Dupre. I conclude on a more general note by questioning the relationship between taxonomic scientific practice and philosophical doctrines of natural kinds. (..) taxonomic pluralism - e.g. Vega in the Lyra constellation, commonly known as an "A0 V" star, that is, a relatively hot, slightly bluish "main sequence dwarf", Vega is also classified by astrophysicists observing in the far infrared part of the electromagnetic spectrum as a "1n-18" star, that is, a star with no remarkable spectral feature in this domain of wavelength, and for those studying how the light emitted by a star varies, Vega is known as a "Delta scuti" type of star, that is, a kind of pulsating variable star. (..) 2.1. Stellar Structuralism. Stars are grouped together in a class when they exhibit features "similar enough" to those of a standard star picked up to define the class. But of course, not just any feature will do as a similarity parameter. (..) The main lines of disagreement between various standpoints on natural kinds and classifications (i.e., monism, pluralism, realism, antirealism, and some combinations of those four) boil down to divergent views of what privileged means [in "priviliged features"]. (..) Astrophysicists want to know how stars form, evolve, and disappear. Their theoretical understanding of the behavior of gaseous spheres tells them that parameters such as temperature, density, or mass loss are determinant parameters in stellar evolutionary processes, whereas proper motion or distance from the earth are not; hence, we have their choice of the former, and not the latter, as taxonomic parameters. (..) 2.2. Wavelength Dependency - predominance of the visible part of the electromagnetic spectrum - which is a contigency determined by what humans can see. (..) 2.3. Resolution Dependency - For a given set of similarity parameters, as to whether a star is similar enough to the standard, one depends on the resolution of the observations used by the classifiers. (..) 2.4. Vagueness - Another straightforward consequence of stellar structure being defined by continuous parameters (such as temperature and density) is the vagueness of the similarity relations between two stars. stellar kinds resemble to a certain extent molecular kinds - due to internuclear distances and angles between bonds that vary continuously. (..) 2.5. Taxonomic Nomadism - a star’s classification is not a permanent matter: properties on which stellar classifications are based are transitory properties. To learn about the different evolutionary phases - in particular, how long a star will spend in each of these phases - astrophysicist compare how many stars belong to each of the associated stellar kinds. From the statistical repartition of stars into kinds, they are then able to derive information on various physical states along an evolutionary path. (..) Two stars may have similar structural properties governing their visible spectra (say, similar temperature), but they may differ significantly by structural properties governing their UV spectra (e.g., they may have different mass loss). (..) Stellar taxonomy should please pluralists on several grounds. Pluralists usually draw on the diversity of biological taxa to dismiss the monist quest for a unique correct way of classifying things (Dupre´ 1993; Kitcher 2001), whereas physics and chemistry are widely considered as monist friendly (see, e.g., Ellis [2002] on the monist side and Slater [2005] on the pluralist side). (..) clearly distinguish between two claims: the claim that the appropriate kind-membership conditions are structural conditions and the claim that there exists a single kind-membership condition (or set of them) that is central to explaining a large variety of behaviors. Structuralism and monism happen to both hold for chemical elements (and, incidentally, neither holds for biological species), but the stellar case shows us that they do not always go hand in hand: structuralism does not favor taxonomic monism over taxonomic pluralism. (..) Hendry 2006,865: the interests that govern the chemistry classifications are more unified; nuclear charge does not have any serious competitor as a grouping criterion for chemical elements. (..) stellar taxonomy further undermines the monist thesis (already significantly weakened by arguments drawing on the diversity of biological taxa) that there exists only one correct way of classifying things that science aims at discovering. Second, it shows that one may have several crosscutting ways of grouping things, all based on the same type of kind-membership condition, to wit, structural. (..) As suggested by Slater (2005), taxonomic monism may be split into two claims: a claim about classification - there exists one unique way of classifying things - and a metaphysical claim about the objectivity and the uniqueness of the distinctions demarcating natural kinds. So far, only classificatory monism has been dismissed. (..) Note that metaphysical monism is stronger than realism. It does not only state that there exist real, mind-independent similarities and differences in nature; it also claims that there exists a uniquely privileged set of such similarities and differences, where "privileged" is conceived in an essentialist way. traditional candidates for essential properties being structural properties (e.g., genetic structures for biological species, molecular structures for chemical substances). (..) stellar kinds do not have sharp boundaries, and a star may belong to more than one kind - the quest for essential properties inevitably leads us to "individualism" about natural kinds, that is, to count as many kinds of stars as there are stars. Denying as I do that there are objective distinctions between stellar kinds does not mean that taxonomic features are not real, mind-independent features of the world. I do take temperature and density as objective features of the stellar world: no doubt, differences in terms of structural properties between individual stars are discovered, rather than conventionally marked. But given the continuous variation of taxonomic parameters from one star to another, and the resulting vagueness and lack of infimic kinds, differences demarcating stellar kinds are not discovered but conventionally marked. Realism about stellar kinds is untenable. (..) So, are stellar kinds natural kinds? Well, in light of the previous discussion, the answer is a ringing no on any realist reading of the notion, be it monist (essentialist) or pluralist (promiscuous realism a` la Dupre). The appropriate reading of the term "privileged" in the astrophysical context is therefore an epistemological, interest-depending one. (..) required: a thorough analysis of the epistemic virtues of stellar kinds in terms of explanation, inductive prediction, and so on. (..) When navigating through the intricate variety of currently competing doctrines of natural kinds - from strong, essentialist ones (Wilkerson 1993; Ellis 1996, 2002) to more or less weaker, nonessentialist ones (Dupre´ 1993, 2002; Boyd 1999; Griffiths 2004; LaPorte 2004) - it is not always clear what their motivations and expectations are as regards existing scientific kinds. (..) what makes certain kinds paradigmatic examples of natural kinds? whatever the answers to those questions are, they will need to be justified by specifying what should be expected from a theory of natural kinds. - commitments about what should be the constraints on those doctrines brought by the kinds defined and successfully used by practicing scientists. |