What are Natural Kinds?
Hawley (Katherine) & Bird (Alexander)
Source: St. Andrews' Website; Philosophical Perspectives 25.1 (2011), 205-221.
Paper - Abstract

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Author’s Introduction (Section 1. Two Questions)

  1. Distinguish two questions about natural kinds1.
    • First, the naturalness question: what, if anything, makes a natural kind2 natural? Perhaps the members of a natural kind3, unlike the members of an arbitrary group, stand in some natural similarity relation to one another. Perhaps they share an essence or some other natural feature. Or perhaps, as conventionalists argue, the boundaries of natural kinds4 do not correspond to distinctions in nature.
    • Second, the kindhood question: what, if anything, makes a natural kind5 a kind? The distinction between natural kinds6 and other natural groupings was noted by Mill (1843: 122–3), who remarked that horses formed a natural kind7 but white things do not. Mill’s point is that, natural similarity among white things notwithstanding, leukocytes, chalks, white vans, clouds, comets, and degenerate (white) dwarf stars are too diverse a group to form a natural kind8. If Mill is right, then investigating the difference between grue and green, for example, will not fully illuminate natural kinds9, for neither green things nor grue things form a natural kind10.
  2. The naturalness question has dominated debate about natural kinds11, at the expense of the kindhood question. One consequence of this dominance is that certain questions concerning the ontology of natural kinds12 have been obscured. Richard Boyd, for example, explicitly understands questions of the form ‘is such-and-such a kind real?’ to be questions concerning naturalness and integration with inductive practices (‘accommodation’).
  3. But there are two tasks:
    • to distinguish natural from unnatural similarities – which will help explain ‘accommodation’ – and also
    • to understand the metaphysical status of natural kinds13, distinguishing the natural similarities that underpin natural kinds14 from the natural similarities that do not.
    In this paper, we take the kindhood question seriously, and we examine how best to develop a realist view of natural kinds15. In particular, we will articulate a view of natural kinds16 as complex universals17. We do not attempt to argue for the existence of universals18. Instead, we will argue that, given the existence of universals19, and of natural kinds20, the latter can be understood in terms of the former, and that this provides a rich, flexible framework within which to discuss issues of indeterminacy, essentialism, induction, and reduction.

  1. Two Questions
  2. The Kindhood21 Question
  3. What are Complex Universals27?
  4. Natural Kinds28 as Complex Universals29
    … 4.1 Precise Structural Kinds
    … 4.2 Precise Conjunctive Kinds
    … 4.3 Indeterminate Kinds
    … 4.4 Exception-permitting Kinds?
  5. Complex Universals30 and Homeostasis
  6. Essences of Natural Kinds31
  7. Conclusion: We have not done enough to persuade the determined sceptic about natural kinds32, or about universals33 to come over to our point of view. However, we have begun to explore the potential rewards of identifying natural kinds34 with mereologically complex universals35, and thereby drawing upon a range of better-understood ideas about mereologically complex ordinary objects. Incorporating kinds into our ontology this way enables us to recognise their significance without having to posit an entirely new category of being.


See Link (Defunct).

In-Page Footnotes

Footnote 21: The authors reject out of hand the nominalist assertion that there are no such things as Natural kinds.

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