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We talk as if there are natural kinds and in particular we quantify over them. We can count the number of elements discovered by Sir Humphrey Davy, or the number of kinds of particle in the standard model. Consequently, it looks at first sight at least, that natural kinds are entities of a sort. In the light of this we may ask certain questions: is the apparent existence of natural kinds real or an illusion? And if real, what sort of entity are natural kinds? Are they sui generis? Or can they be identified with or reduced to some other kind of entity? In this essay I shall look at possible reasons for asserting that either kinds are no sort of entity, or, if they are entities, their existence is equivalent to some fact not involving kinds. Richard Boyd seems to take the view that the apparent existence of natural kinds is an illusion.

The vision of natural kinds that is most common in the modern philosophy of biology, particularly with respect to the question whether species and other taxa are natural kinds, is based on a revision of the notion by Mill in A System of Logic. However, there was another conception that Whewell had previously captured well, which taxonomists have always employed, of kinds as being types that need not have necessary and sufficient characters and properties, or essences. These competing views employ different approaches to scientific methodologies: Mill’s class-kinds are not formed by induction but by deduction, while Whewell’s type-kinds are inductive. More recently, phylogenetic kinds (clades, or monophyletic-kinds) are inductively projectible, and escape Mill’s strictures. Mill’s version represents a shift in the notions of kinds from the biological to the physical sciences.

Natural kinds have been a constant topic in philosophy throughout its history, but many issues pertaining to natural kinds still remain unresolved. This paper considers one of these issues: the epistemic role of natural kinds in scientific investigation. I begin by clarifying what is at stake for an individual scientific field when asking whether or not the field studies a natural kind. I use an example from life science, concerning how biologists explain the similar body shapes of fish and cetaceans, to show that natural kinds play a central epistemic role in scientific explanations that cannot be delegated to other explanatory factors. A task for philosophy, then, is to come up with a theory of natural kinds that adequately accounts for the epistemic role of natural kinds in science. After having sketched the spectrum of available philosophical theories of natural kinds, I argue that none of the available theories adequately performs this task and that therefore the search is still open for a theory that does.

The aim of this article is to illustrate how a belief in the existence of kinds may be justified for the particular case of natural kinds: particularly noteworthy in this respect is the weight borne by scientific natural kinds (e.g., physical, chemical, and biological kinds) in (i) inductive arguments; (ii) the laws of nature; and (iii) causal explanations. It is argued that biological taxa are properly viewed as kinds as well, despite the fact that they have been by some alleged to be individuals. Since it turns out that the arguments associated with the standard Kripke/Putnam semantics for natural kind terms only establish the non-descriptiveness of natural kind terms and not their rigidity, the door is open to analyze these terms as denoting traditional predicate-extensions. Finally, special issues raised by physical and chemical kinds are considered briefly, in particular impurities, isotopes and the threat of incommensurability.

The Philosophical search for Natural Kinds is motivated by the hope of finding ontological categories that are independent of our interests. Other requirements, of varying importance, are commonly made of kinds that claim to be natural. But no such categories are to be found. Virtually any kind can be termed 'natural' relative to some set of interests and epistemic priorities. Science determines those priorities at any particular stage of its progress, and what kinds are most 'natural' in that sense is always a real and lively scientific question. The general philosophical problem of scientific realism is also real; but between the scientific and the metaphysical, the Problem of Natural Kinds' sits otiose.

In this paper I discuss a central objection against diseases being natural kinds—namely, that diseases are processes or transitions and hence they should not be conceptualized in the ‘substantish’ framework of natural kinds. I indicate that the objection hinges on conceiving disease kinds as phase kinds, in contrast to the non-phase, natural kinds of the exact sciences. I focus on somatic diseases and argue, via a representative comparison, that if disease kinds are phase kinds, then exact science kinds are phase kinds as well. On the other hand, if exact science kinds are non-phase kinds, then disease kinds are non-phase kinds as well. This objection should thus be rejected, under a certain caveat, though. If natural kind membership has an influence over the diachronic identity of kind members, then it is possible, in principle, to draw the phase/non-phase distinction such that an ‘ontological gap’ lies between medical kinds and exact science kinds. I show further that this caveat is unavoidable even in relation to substantive universals and ‘essential’ properties—two controversial, strong features that were traditionally associated to natural kinds.

The thought that diseases form natural kinds tends not to sit well with the essentialist treatment of natural kinds. The essentialist’s candidates for the essences of diseases—etiological properties—rarely satisfy the essentialist’s requirement that they be necessary and sufficient for membership within the kind. Consequently philosophers of medicine have tended to back away from treating diseases as natural kinds. However, this retreat was too hasty: there are good reasons for thinking that diseases form natural kinds. The problem lies with the essentialist treatment of natural kinds, and not with treating disease as forming natural kinds. A similar revolution has taken place within the species debate, where the notion of natural kind has been ‘taken back’ from the essentialist. Borrowing the revised treatment of natural kinds from the philosophers of species and modifying it slightly, I offer a proposed treatment of disease kinds in terms of homeostatic property clusters.

Natural kinds are often contrasted with other kinds of scientific kinds, especially functional kinds, because of a presumed categorical difference in explanatory value: supposedly, natural kinds can ground explanations, while other kinds of kinds cannot. I argue against this view of natural kinds by examining a particular type of explanation—mechanistic explanation—and showing that functional kinds do the same work there as traditionally recognized natural kinds are supposed to do in “standard” scientific explanations. Breaking down this categorical distinction between traditional natural kinds and other kinds of kinds, I argue, delivers two goods: It provides us with a view of natural kindhood that does justice to the epistemic roles of kinds in scientific explanations. And it allows us to solve a problem that HPC theory, currently one of the more popular accounts of natural kindhood, confronts. † To contact the author, please write to: Center for Philosophy and Ethics of Science (ZEWW), Leibniz University of Hannover, Im Moore 21, 30167 Hannover, Germany; e‐mail: reydon@ww.uni‐hannover.de.

Taxa and homologues can in our view be construed both as kinds and as individuals. However, the conceptualization of taxa as natural kinds in the sense of homeostatic property cluster kinds has been criticized by some systematists, as it seems that even such kinds cannot evolve due to their being homeostatic. We reply by arguing that the treatment of transformational and taxic homologies, respectively, as dynamic and static aspects of the same homeostatic property cluster kind represents a good perspective for supporting the conceptualization of taxa as kinds. The focus on a phenomenon of homology based on causal processes (e.g., connectivity, activity-function, genetics, inheritance, and modularity) and implying relationship with modification yields a notion of natural kinds conforming to the phylogenetic-evolutionary framework. Nevertheless, homeostatic property cluster kinds in taxonomic and evolutionary practice must be rooted in the primacy of epistemological classification (homology as observational properties) over metaphysical generalization (series of transformation and common ancestry as unobservational processes). The perspective of individuating characters exclusively by historical-transformational independence instead of their developmental, structural, and functional independence fails to yield a sufficient practical interplay between theory and observation. Purely ontological and ostensional perspectives in evolution and phylogeny (e.g., an ideographic character concept and PhyloCode’s ‘individualism’ of clades) may be pragmatically contested in the case of urgent issues in biodiversity research, conservation, and systematics.

Many realists on kinds deem it highly controversial to consider artefact kinds real kinds on a par with natural ones. There is a built-in tendency in realism to conceive of artefact kinds as merely a conventional classification used for practical purposes. One can individuate three main different approaches characterizing real kinds and accordingly three different types of arguments against viewing artefact kinds as real kinds: the metaphysical, the epistemological and the semantic arguments. The aim of this contribution is to undermine the thesis that it is possible to trace a clear distinction between artefacts and natural kinds in each of these approaches. As a consequence there are no metaphysical, epistemological and semantic bases for claiming that artefact kinds as opposed to natural ones are not real kinds.