Natural kinds are objective groupings of objects of which we can make reliable inductive inferences, such as gold, lions and pyrite. Although the metaphysical debate on kinds is as old as Aristotle’s views on genera and species, its modern version comes from the work by Kripke and Putnam on natural kind terms.
Instances of a kind belong to it in virtue of sharing some common properties. Essentialists argue that these properties are given by general essences, properties which are had necessarily by the members of the kind and that fix also their existence and persistence conditions. Cluster theorists argue that these properties simply tend to co-occur, possibly because of causal relations that hold between them.
Some of my publications in this area are:
- Belastegui, J. (2022): Natural Kind Semantics for a Classical Essentialist Theory of Kinds. The Review of Symbolic Logic, 17, 2, 509-545. DOI: 10.1017/S1755020322000351
In classical (modal) logic, a property is represented extensionally as the set of objects that have that property (or as a function from possible worlds to sets of objects). However, in the case of natural kind essentialism, this representation is missing something important if used to represent kinds. A natural kind essentialist believes that natural kinds have general essences, which correspond to certain properties that all the instances of the kind have necessarily de re.
In this paper I outline an essentialist theory of kinds in first-order modal logic which takes as axioms the main theses of essentialism, such as Kripkean individual essentialism (if an object belongs to a kind, it does so necessarily) and the claim that the membership conditions of kinds are fixed by their general essences. I also provide a lattice-theoretic semantics for this theory that represents kinds as rigidly designated pairs of sets. Whereas one set is the extension of the kind, the other set is the general essence of the kind. I use this semantics to discuss the Hierarchy condition from the literature on kinds, a principle by Kant on the duality between extension and essence and also operations of negation and specific difference between kinds.
- Belastegui, J. (2021): The Resemblance Structure of Natural Kinds: A Formal Model for Resemblance Nominalism. PhD Dissertation, Addi Open Repository, University of the Basque Country.
This was my PhD thesis, where I applied a Carnapian approach to the framework of conceptual spaces to study natural kinds.
The external structure of kinds is given by their species-genus relations, the relations of one kind being more specific than another kind. The internal structure of kinds is given by the properties shared by the members of the kind.
I proposed to reconstruct formally these two features of kinds based on the assumptions of Resemblance Nominalism, by making use of some ideas from Carnap’s Aufbau work on the relation between properties and similarity. The properties shared by the members of the kind were reconstructed as sets of objects in similarity relations, making use of the method of quasi-analysis.