Draft:Homeostatic property cluster theory

Homeostatic property cluster theory (HPC theory) is an account of natural kinds in the philosophy of science, introduced by Richard Boyd as a non-essentialist alternative to views that define kinds by necessary and sufficient conditions. On this account, many natural kinds are characterised by clusters of properties whose co-occurrence is maintained by causal mechanisms, even though no single property need be shared by every member and the boundaries of the kind may be vague. Boyd developed the account chiefly with biological species in mind, and it became a widely discussed alternative to essentialism in debates over natural kinds.

The theory has been criticised for the difficulty of individuating the causal mechanisms it requires. P. D. Magnus has argued that being a natural kind is neither necessary nor sufficient for being an HPC, and that the account is better understood as describing the ontological basis of some natural kinds rather than defining natural kinds as such. Matthew Slater has proposed a refinement, the stable property cluster (SPC) account, which replaces the mechanism requirement with a more general notion of stability.

Background

Traditional essentialism about natural kinds holds that each kind is defined by a real essence statable in terms of necessary and sufficient conditions for membership. This approach faces difficulties when applied to biological species. Species members vary considerably in their properties; no single genetic, morphological, or behavioural trait is shared by all and only the members of a given species, and species boundaries are often not sharp. Boyd argued that these features are not defects of biological taxonomy but reflect the genuine structure of the natural world, and that an account of natural kinds was needed that could accommodate them.

Theory

Core formulation

Boyd set out the HPC account in a series of papers beginning in the early 1990s, with the fullest statement in his 1999 chapter "Homeostasis, Species, and Higher Taxa". As Magnus summarises it, "an HPC is a structured repetition of properties in the world, maintained by an underlying causal process". Boyd's formulation has several components:

1. There is a family of properties that are contingently clustered in nature, in the sense that they co-occur in an important number of cases.
2. This co-occurrence is "at least typically, the result of what may be metaphorically (sometimes literally) described as a sort of homeostasis". Either the presence of some properties tends to favour the presence of others, or there are underlying mechanisms or processes that maintain the cluster, or both.
3. The homeostatic clustering of the properties is causally important: significant effects are produced by the joint occurrence of many of the properties together with the underlying mechanisms.
4. The kind term has no analytic definition. Rather, the property cluster together with the mechanisms that maintain it provide a natural definition. Which properties and mechanisms belong in the definition is an a posteriori theoretical question, not an a priori conceptual one.

Imperfect homeostasis is normal: some members of a kind may display some but not all of the properties in the cluster, and some but not all of the relevant mechanisms may be operating. Cases of extensional indeterminacy — borderline cases where no rational consideration dictates whether something falls under the kind — are an expected and irremovable feature, not a sign that the classification is defective.

The accommodation thesis

Central to Boyd's account is what he calls the accommodation thesis: that the naturalness of natural kinds consists in the contribution that reference to them makes to successful induction and explanation. Kinds are natural insofar as they allow scientists to "accommodate" their classificatory practices to the causal structure of the world. In Boyd's formulation, "the naturalness of natural kinds consists in their aptness for induction and explanation". Magnus notes that this makes the accommodation thesis an answer to what he calls the taxonomy question — what distinguishes natural kinds from arbitrary categories — while HPCs themselves answer the ontology question of what holds particular kinds together.

Vagueness and historicity

Two consequences of the HPC account distinguish it from essentialist views of natural kinds.

First, HPC kinds are inherently vague. Because no single property is individually necessary for kind membership, the boundary of the kind cannot be drawn sharply. Boyd argued that no refinement of usage that replaces a vague kind term with a more precise one can preserve the naturalness of the kind: any such refinement would either treat as important distinctions that are irrelevant to causal explanation or ignore similarities that are important to it.

Second, HPC kinds are historical. The property cluster that defines a kind is not individuated extensionally (by its current membership) but like a historical entity: certain changes over time in the property cluster or in the underlying mechanisms preserve the identity of the kind, while others do not. This means that the properties determining membership in a kind can shift over time while the kind retains its identity — a feature Boyd took to be essential for handling biological species, which evolve.

Boyd noted that the HPC account is self-applying: "The kind natural kind is itself a natural kind in the theory of our inferential practice."

Applications

Biology

Biological species were Boyd's paradigm case for HPC kinds. Members of a species typically share a cluster of morphological, behavioural, physiological, and genetic properties, but none of these properties is individually necessary or sufficient for membership. The co-occurrence of these properties is maintained by homeostatic mechanisms including gene flow within populations, shared developmental pathways, and common ecological constraints. Boyd argued that species are "paradigmatic natural kinds, their historicality and lack of sharp boundaries notwithstanding".

The species case also illustrates the accommodation thesis. Biologists have proposed competing species concepts — the biological species concept based on interbreeding, phylogenetic species concepts based on shared ancestry, and ecological species concepts based on adaptive zone — each serving different explanatory projects. Boyd argued that this plurality is expected on the HPC account: different ways of demarcating species correspond to different accommodation demands and can be equally legitimate, since each tracks causally relevant structure for its own purposes.

Other proposed applications

Beyond biology, the HPC account has been discussed in relation to kinds in psychology, psychiatry, and chemistry. In the philosophy of language, Miller (2021) argued that individual words (considered as types rather than tokens) are HPC kinds, drawing an explicit analogy with biological species. The question of whether and how the account extends beyond biology has been a recurring theme in the literature.

Criticisms and refinements

Scope of the account

Magnus (2014) argued that the HPC account is often misconstrued as a definition of natural kinds, when it is better understood as describing the ontological basis of some of them. He distinguished two questions: the taxonomy question (what distinguishes a natural kind from an arbitrary category?) and the ontology question (what features of the world hold a particular kind together?). The HPC account, Magnus argued, answers only the ontology question, and only for a subset of natural kinds.

The argument turns on cases that pull apart in each direction. Electrons are natural kinds but not HPCs: their properties are not maintained by homeostatic mechanisms but hold as a matter of nomic necessity. Conversely, some trivial HPCs — clusters of properties held together by causes — do not constitute natural kinds in any scientifically useful sense. Magnus concluded that Boyd's own formulation already accommodates this point, since Boyd described HPCs as "a class of natural kinds" and "an important family of natural kinds" rather than equating the two.

The mechanism problem

Several critics have raised concerns about the role of causal mechanisms in the HPC account. Craver (2009) argued that individuating mechanisms is itself partly conventional: "human perspectives and conventions enter into judgments about how mechanisms should be typed and individuated". If the mechanisms that sustain a property cluster cannot be objectively delineated, then the kinds they define may depend partly on human convention, undermining the realist credentials of the account. Craver noted that "one can be led to lump or split the same putative kind in different ways depending on which mechanism one consults".

Onishi and Serpico (2022) surveyed attempts to formulate HPC theory without homeostatic mechanisms and argued that such attempts face significant costs, either weakening the account's explanatory power or collapsing into a different kind of theory altogether.

Stable property cluster account

Slater (2015) proposed the stable property cluster (SPC) account as a refinement of HPC theory. The SPC account shifts the focus from the mechanisms that maintain a property cluster to the stability of the cluster itself. Slater argued that "the HPC view can be made clearer by focusing on the special sort of stability that a cluster of properties might have whereby it is suitable for inductive and explanatory purposes, instead of focusing on something causing that stability".

The SPC account was motivated by several concerns about the mechanism requirement. Some scientifically important kinds are associated with property clusters whose stability is not plausibly maintained by causal homeostatic mechanisms. Fundamental particles possess their properties as a matter of nomic necessity, not because of any sustaining mechanism. In biology, sibling species may retain stable property clusters through causal isolation rather than active homeostasis — what Slater compared to metastability in chemistry. The SPC account treats essentialist kinds as a limiting case — the extreme of maximal stability — rather than as a categorically different type of kind.

See also

• Natural kind
• Essentialism
• Philosophy of biology
• Species problem
• Cluster concept

Further reading

• Boyd, Richard (1999). "Kinds, Complexity and Multiple Realization". Philosophical Studies. 95 (1–2): 67–98. doi:10.1023/a:1004511407133.
• Ereshefsky, Marc; Reydon, Thomas A. C. (2015). "Scientific Kinds". Philosophical Studies. 172 (4): 969–986. doi:10.1007/s11098-014-0301-4.
• Khalidi, Muhammad Ali (2013). Natural Categories and Human Kinds: Classification in the Natural and Social Sciences. Cambridge: Cambridge University Press. doi:10.1017/CBO9780511998553. ISBN 978-0-511-99855-3.
• Kornblith, Hilary (1993). Inductive Inference and Its Natural Ground: An Essay in Naturalistic Epistemology. Cambridge, MA: MIT Press. doi:10.7551/mitpress/3730.001.0001. ISBN 978-0-262-27746-4.
• Millikan, Ruth Garrett (1999). "Historical Kinds and the "Special Sciences"". Philosophical Studies. 95 (1–2): 45–65. doi:10.1023/A:1004532016219.

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