Abstract
This paper maps the landscape for a range of views concerning the metaphysics of natural kinds. I consider a range of increasingly ontologically committed views concerning natural kinds and the possible arguments for them. I then ask how these relate to natural kind essentialism, arguing that essentialism requires commitment to kinds as entities. I conclude by examining the homeostatic property cluster view of kinds in the light of the general understanding of kinds developed.
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Notes
See Hacking (1991) for a historical survey.
‘Naturalism’ can mean a fair variety of things in philosophy (Papineau 2009). One quite general understanding is that a (methodologically) naturalistic approach to Xs means that the truth about Xs can and can only be discovered by using the methods of science. Regarding natural kinds that means that science can discover which natural kinds there are and what they are like (for many kinds if not for all), and that we cannot do this a priori (which we could if kinds were entirely conventional).
In this paper I do not address questions concerning the structure of natural kind classifications. Do natural kinds always come in hierarchies? If so, does any given natural entity belong at most to one hierarchy (monism)? Or could an entity belong to more than one system of natural classification?
I say ‘typically’, because I do not wish to assert that kind membership is an intrinsic property of an entity. Often it is, but membership of a biological kind may well depend on origin, which will be a relational rather than intrinsic property of an object.
Quine (1969, pp. 137–138) does remark that, “In general we can take it as a very special mark of the maturity of a branch of science that it no longer needs an irreducible notion of similarity and kind.” But Quine is not referring to the elimination of kinds specifically but rather to the elimination of qualitative properties, such as colour, in favour of quantitative ones, such as mass.
Much of the blame for the tendency to ignore the kindhood question in favour of the naturalness question can be placed on the habit, found in Quine’s article for example, to take the discussion of Goodman’s new riddle of induction to be central to understanding natural kinds. But, as Mill would have pointed out, even if green things are naturally similar and grue things are not, it remains the case that green things do not form a natural kind.
I do not ask whether there is an difference between characterization of essence in terms of identity or nature: no such difference will affect my argument.
Not in the Cartesian sense that the content of the essence of God implies that God exists. Rather, the fact that some x has an essence implies that x exists.
Isn’t the round square essentially square? I see no reason to agree. I have argued that the nature of non-existent entities is obscure, and the nature of impossible entities is even more obscure.
Of course, this leaves it open that it is essential to the concept unicorn that it is the concept of a mythical creature. But that is consistent with the essence implies existence thesis, since that thesis would imply on the existence of the concept unicorn, which is not in doubt.
Abbott (1997, p. 312) points out that to “get Putnam’s results one does not need to use terms like ‘reference ’ or ‘extension’ ...All one need to ask is whether ...such-and-such a substance would be water”.
Devitt notes in response to criticism by Schwartz (2002, p. 275) that he does not claim that it is the function of rigid designation to distinguish natural from non-natural kinds. However, this criticism is different, since Devitt does regard it as a secondary function of an account of rigid designation that it explain, even if only superficially, the modal features of kind and general terms. Thus while his view does not need to account for the distinction between natural and non-natural kinds, it ought to identify the distinction between those general terms which are associated with essences and those which are not.
It is important to note that species membership is not a matter of genotype (cf. Ridley (2003); but see also Devitt (2008), who denies this). It is true that members of a species will be genetically similar, but there will often be no unique genetic character that they all share but is not shared by anything outside the species—particularly if we consider members of the same species that come from different points in the species’ evolutionary history. In particular, a certain mutation may in due course lead to a speciation event, but the arrival of that mutation cannot itself be regarded as the arrival of a new species, at least not for sexually reproducing species. The organism with the new mutation will need to breed with organisms without it. If the mutated organism is the first member of the new species, then its offspring from mating with a non-mutated member of the old species will be members of which species? Furthermore, since mutations are frequent, the arrival of not just any mutation will create a new species; it will have to spread throughout a significant subpopulation, leading to splitting of the original population. If the mutation is the speciation event, then whether or not it counts as a speciation event will depend on subsequent events—the spreading of the mutation and the splitting of the populations. In which case whether or not an organism belongs to species S at t will depend on events occurring later than t, which seems at least undesirable.
The cross-temporal counterparts are not the four-dimensional objects; rather they are time-slices. Note that David Lewis’s motivation for transworld counterparts, the problem of accidental intrinsics is the analogue of his reason for adopting time-slices, the problem of temporary intrinsics (Lewis 1986b, pp. 202–204, 210–220).
Another example of items that change their kind might be stem cells, which are able to become cells of a specific kind that they did not previously belong to (I owe this example to Peter Farleigh). In particular, induced pluripotent stem cells start out as adult stem cells and are reprogrammed to become pluripotent stem cells and thereafter become specific specialised cells of widely varying kinds, depending on their environment (here I owe thanks to an anonymous referee). Devitt himself accepts that not all natural kind terms apply rigidly, citing Schwartz’s example of ‘frog’: an individual that is a tadpole may become a frog, but might not have done. I don’t mention this example, because it is not obvious that frogs (in this sense) form a natural kind, since ‘frog’ is a phase sortal. If some phase sortals can be natural kinds, then there are many counterexamples, including not only biological phases (pupa, embryo, etc.) but also stellar phases.
Or if one does want to allow existence to the kind of which wedding rings are mostly made, as an abundant property perhaps, then the argument allows us to distinguish those from the natural kinds, since the essences of the latter are of a different sort.
See Bird (2011) for a more detailed discussion of this point.
Indeed, the proposal considered make kinds look like the senses or intensions of kind terms, whereas the kinds themselves should be the referents of such terms.
An on-off property can be accommodated by their characteristic function: 0 for lacking the property, 1 for possessing it.
Note that while such a view sees kinds as complex universals rather than particulars, the analogue with concrete particulars means that it can accommodate many of the insights that the view of species as individuals (Hull 1976).
This conclusion does not exclude the possibility that alternatives to the HPC view may also cohere with the metaphysical conclusions reached here.
Although I presented the answers to subsequent questions as building on positive answers to earlier questions, leading to the a maximally committed metaphysics of kinds, there may be positions that allow for a combination of negative responses to earlier questions with positive answers to later ones [e.g. a social constructivist about ontology might answer ‘no’ to (Q1) and ‘yes’ to (Q3)], and so a full account of all the possibilities for a metaphysics of natural kinds may see these questions as providing a matrix rather than a decision tree, as Huw Price has suggested to me.
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Acknowledgments
I am grateful to audiences at the Inland Northwestern Philosophy Conference 2008, at the University of Idaho, the Austrian Ludwig Wittgenstein Society’s annual conference 2008 in Kichberg am Wechsel, the University of Sheffield, the Cambridge Serious Metaphysics group, Saint Louis University, the University of Melbourne, the University of Sydney, the Australian National University, IHPST in Paris, the Metaphysics of Science conference in Nottingham 2009, and the Metaphysics of Science conference at Kyung Hee University, South Korea in 2010, as well as to anonymous referees for Synthese.
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Bird, A. The metaphysics of natural kinds. Synthese 195, 1397–1426 (2018). https://doi.org/10.1007/s11229-015-0833-y
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DOI: https://doi.org/10.1007/s11229-015-0833-y