Are Species Real? An Essay on the Metaphysics of Species
The title ('Are Species Real?') is the question my book attempts to answer. My answer (spoiler alert!): yes; or more precisely: yes and no. I'll explain presently why this is not a hollow cop-out. First, it's worth noting the attraction to the view that species are indeed real features of the world. Species are deeply embedded in both scientific investigation and everyday enthusiasm. Our species goes to considerable effort and expense to watch, document, count, breed, cultivate, hunt, and conserve other species. Scientists often report discovering new species, estimating that there are ten million distinct species presently on earth (and many more that have disappeared).
But what are species? Do everyday intuitions that species are real stand up to philosophical and scientific scrutiny? Two rival accounts of species' reality have dominated the discussion: that species are natural kinds defined by essential properties and that species are individuals. I contend that neither fully accommodates biological practice or require uncontroversial metaphysical premises. One of the central motivations for the species-as-individuals view has been that the view that species are natural kinds saddles us with an expensive ontology of abstract objects (sets) which cannot accommodate the changing natural world. Ironically, my primary criticism of the species-as-individuals thesis is that, by treating species as objects, its defenders incur demands for metaphysical precision that are seriously out of step with biological practice.
I shall argue for an alternative approach to the metaphysics of species. Species are real, I say, not in virtue of there being particular sets or individuals in the world, but in virtue of there being some organisms bearing certain relations to one another. Sentences like 'There are six species of giraffe' are true, not because this sentence refers to six distinct things, but in virtue of some animals sharing, say, particular breeding patterns. There are six populations of organisms that are natural kinds (understood in a non-essentialist way). Thus, I propose to separate the question of whether something is a natural kind from the question of whether it is a set or an individual. A natural kind, in my view, is not an ontological category. Natural kinds can crosscut ontological categories like sets, individuals, properties, relations, events, processes, and so on. Much confusion in the debate about the metaphysics of species has devolved from failing to separate questions about species' metaphysical category from the question of whether they are natural kinds. I shall show that we can treat species names as plural referring expressions — referring not to individual sets (abstract objects), but to collections of things — and yet nevertheless treat these expressions as names of natural kinds.
This raises a key question: what is a natural kind? By refusing to associate natural kinds with a particular ontological category, I free myself up make this a partly epistemic question. In something like the spirit of Goodman and Boyd, I suggest that the natural kinds are those things (be them individuals, sets, collections, whatever) which play characteristic roles in our inductive and explanatory practices. But while there is no doubt a pragmatic element to these practices, it is something about the metaphysics of these collections that suits them to this role. I develop a novel conception of natural kinds closely allied with Boyd's Homeostatic Property Cluster (HPC) conception. But whereas Boyd says that the relevant clusters of properties must be maintained by homeostatic causal mechanisms, I require only that the properties be stable across various counterfactual perturbations. Which perturbations? This is where the pragmatics of explanation and induction reenter the picture. The last third of the book will be devoted to articulating the account — what I call the "Stable Property Cluster" (SPC) account of natural kinds — testing it against case studies, and exploring several of its biological and metaphysical consequences.