Intrinsic property: the necessary property/ies that make a thing what it is. Usually it is considered to be part of the things microstructure but not necessarily. E.g. An intrinsic property of a car is that it has 4 wheels and an engine. An intrinsic property of water is that its chemical structure is H20.
Essentialism (about kinds): The idea that things have essences that make them what they are. This is often used interchangeably with intrinsic property. For example, what makes the kind 'dog' is that all dogs have the essence of "doggyness". This could also be explained by properties like characteristic behaviours, DNA, or form. Aristotle classically declared that the essence of 'man' is rational thought.
Okasha takes essentialism to be the position of Kripke, Putnam, and Wiggins—i.e., that the concept of kind essentialism used in the hard sciences also applies to biological kinds. Okasha suggests Putnam and Kripke (PK) are mistaken to apply their brand of essentialism to biological kinds because (a) of facts about evolutionary biology that don't apply to the hard sciences, and (b) the purpose of classification in biology is different from that in the hard sciences. However, it is not all doom and gloom for the PK model: if instead of demarcating kinds according to intrinsic properties we use relational properties, aspects of the PK model can be salvaged.
The philosophical origins of the PK essentialist model can be traced back to Locke's distinction between the nominal and real essence of a kind. The nominal essences are the macro-properties of an object that we pick out to determine the group to into which we place it. The standard example is that the nominal essence of gold is that it is shiny, metallic, yellow, and malleable. The particular properties we settle on to define kinds are conventional; that is, they are not dependant on anything intrinsic to the particular things we are classifying, rather they are selected based on utility and/or accidental facts about our perceptual system. For example, we could have grouped objects according to gross size and texture, but this would not have served any useful purpose.
In contrast to nominal essence there is the real essence which is the (intrinsic) hidden underlying microstructure which is causally responsible for nominal essences. If we could access the real essence of objects, we'd be able to group them according to their intrinsic properties/”hidden structure” and therefore their metaphysically real (as opposed to conventional) kinds. Locke didn't anticipate our having “microscopic eyes” to actually identify real essences, so he supposed all kinds would be nominal.
Of course, science has progressed to the point in the hard sciences where microstructure can be identified, as so objects can be classified according to (real) kind. PK natural kinds emerge out of this reality. The standard examples of PK essentialist kinds are “gold is having the atomic number 79” and “water is having the chemical composition H20”. So, determining whether something is a kind is an empirical matter. For example, to determine whether H20 is a kind is a matter of verifying that all samples of water have the molecular structure H201.
Arguments Against Using the PK Model for Biological Kinds
Okasha suggests two main lines of argument that lessen the probability that PK essentialist notion of kinds is applicable to evolutionary biology. The first line of argument is empirical which can be generally framed by referring back to Locke's observation that there is no principled way to distinguish between accidental and intrinsic qualities. Given that evolutionary theory doesn't restrict the possibility of changes (i.e., mutations, meiosis, and genetic recombinations) to any aspect of an group of organisms' phenotypic or genetic properties, it seems unlikely that a kind could have an immutable intrinsic essence. With no necessary enduring intrinsic property (that isn't also shared by other kinds), what type of property could defines the kind?
The conceptual argument again relates back to Locke's operationalism. Even if a set of (genetic/phenotypic/genotypic) properties were shared by all members of a kind and by no non-members, we would not consider having these internal properties necessary to membership. Suppose two members of a species produce an offspring lacking in one of the essential properties. We would probably still group the offspring with its parents.
So, it looks like the (internal) essentialist model for kinds doesn't fit well with the ephemeral nature of species in evolutionary biology. Should we then completely abandon the PK model in relation to biology? Okasha suggests that the PK model is still applicable to biological kinds so long as we relinquish the requirement that essential properties of kinds be intrinsic and instead replace them with relational properties.
Relational Kinds: Retooling the PK Model for Biology
A Relational property in this context means the essential property relates x to other xs. The relation is the property that tells us “in virtue of what organism x is a member of kind y”. In addition, an essential relational property cannot be shared by non-xs. Biologists use four basic relational properties to define species concepts: phenetic, interbreeding, ecological niche, and phylogenetic. While all 4 methods have their weaknesses, the phenetic concept is considered the weakest because it suffers from the same problems as internal essentialist concepts. Furthermore, a peculiarity of the relational species concepts is that two organisms could be molecule for molecule duplicates, but if they don't the share the relational kind property, they are considered to be of different species3.
It seems that with the substitution of relational for intrinsic properties, the PK model can be applied to biological kinds after all. The PK model can maintain the semantic role of kinds terms because now we can say that speakers who use the terms are intending to refer to some essential property beyond superficial appearances. Unfortunately, the applicability ends here because the PK model also implies that the essential property of a kind (its hidden structure) is also causally responsible for its superficial properties. While this is true of chemical kinds, this isn't necessarily the case for biological kinds. An organism's belonging to a particular chunk of the genealogical nexus (or occupying an ecological niche, etc...) isn't the proximal cause of its superficial properties—there is only an indirect causal relationship.
For these reasons Okasha concludes that the PK model is only half-right when applied to biological kinds. On the PK model essences play both a semantic and causal/explanatory role, but “there is no a priori reason why the same thing should play both of these roles4.” While I agree with Okasha here I think there is something a little disconcerting about decoupling these two roles. It appears we lose a degree of objectivity and predictive power.
The fact that there are so many different species-concepts can raise worries about conventionalism. Why should we consider one species-concept over another? Consider that mammals are often grouped according to either phylogenetic or breeding or ecological niche concepts but bacteria are grouped according to degree of variability in section (?) 16 of sRNA. If one strand of bacteria has greater that 1% variability from its “parent” strand then it is considered to be a new species of bacteria5. Clearly, biologists are picking and choosing their species concepts based on what is useful to their research aims (and contingent upon the sophistication of their measurement techniques). If biologist were really classifying according to essential kinds, why isn't there just one concept of species?
This brings us head to head with the final issue I wish to discuss: what is the purpose of a classificatory system? On the PK model it seems that the implicit answer to this question is that kinds are meant to be scientifically useful; viz, “to provide the greatest possible predictively useful generalizations6”. Predictively useful generalizations usually require we know something about the kind's causal structure. But in biology we don't have this information, nor do we need it because it has a different purpose of classification.
The purpose of classification (i.e., species concepts) in biology is to identify “units which we believe play an important role in the evolutionary process7”. Of course, knowing an organism's species (defined in one of the relational concepts) does provide the ability to make predictions about behaviour and morphology, but, again, this is not the primary purpose of having the classification.
To conclude I think it's interesting to consider how species concepts are defined and why one would be used over another in a particular situation. There seems to be an interesting reciprocal relationship between the empirical and the analytic concepts. The species concepts are analytic but they are informed by empirical considerations. Consider the breeding concept. Certainly, whether one group breeds with another is a matter of empirical observation but the decision to define a kind based on a particular concept of inter-breeding rather than another is analytic. However, that particular kind concept will also have been shaped by past empirical observations. At some point, the biologist has to “break in” with a an imperfect concept. The relationship between the analytic and synthetic in the context of natural kinds is something I'm interested in exploring more, so I encourage comments on this issue!