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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.

Saul Kripke, in a series of classic writings of the 1960s and 1970s, changed the face of metaphysics and philosophy of language. Christopher Hughes offers a careful exposition and critical analysis of Kripke's central ideas about names, necessity, and identity. He clears up some common misunderstandings of Kripke's views on rigid designation, causality and reference, and the necessary a posteriori and contingent a priori. Through his engagement with Kripke's ideas Hughes makes a significant contribution to ongoing debates on, inter alia, the semantics of natural kind terms, the nature of natural kinds, the essentiality of origin and constitution, the relative merits of 'identitarian' and counterpart-theoretic accounts of modality, and the identity or otherwise of mental types and tokens with physical types and tokens. No specialist knowledge in either the philosophy of language or metaphysics is presupposed; Hughes's book will be valuable for anyone working on the ideas which Kripke made famous in the philosophy world.

In Naming and Necessity' Saul A. Kripke gives two types of examples of contingent truths knowable a priori. So he disagrees with the first leg of the thesis. As we will see later, his examples depend on the direct designation theory of names. While there have been attempts to provide examples of the contingent a priori that do not depend on that theory, most of those examples should be viewed as expansions, or modifications, of Kripke's examples. Philip Kitcher, for example, gives an interesting example that has nothing to do with theories of names, but is produced using the indexical 'actual'.2 His example, however, is a variation of Kripke's Neptune Type example.' In what follows I will focus on Kripke's two types of examples and modifications of them. I will argue that although both types of example fail, it is possible to modify his Standard Metre example in such a way that we have an example of the contingent a priori.

It has been suggested that the theory of reference advanced by Kripke and Putnam implies, or presupposes, an aristotelian vision of natural kinds and essences. I argue that what is in fact established is that there are degrees of naturalness among kinds. A parallel argument shows that there are degrees of naturalness among individuals. A subsidiary theme of the paper is that the definition of "natural kind term" as "rigid designator of a natural kind" is mistaken. Names and natural kind terms are defined by ostension to a spatiotemporal part of what they ostend. This helps us understand why 'Hesperus' and 'Phosphorus' are cognitively non-equivalent.

Despite the traditional focus on metaphysical issues in discussions of natural kinds in biology, epistemological considerations are at least as important. By revisiting the debate as to whether taxa are kinds or individuals, I argue that both accounts are metaphysically compatible, but that one or the other approach can be pragmatically preferable depending on the epistemic context. Recent objections against construing species as homeostatic property cluster kinds are also addressed. The second part of the paper broadens the perspective by considering homologues as another example of natural kinds, comparing them with analogues as functionally defined kinds. Given that there are various types of natural kinds, I discuss the different theoretical purposes served by diverse kind concepts, suggesting that there is no clear-cut distinction between natural kinds and other kinds, such as functional kinds. Rather than attempting to offer a unique metaphysical account of ‘natural’ kind, a more fruitful approach consists in the epistemological study of how different natural kind concepts are employed in scientific reasoning.

This paper challenges the Kripkean interpretation of a posteriori necessities. It will be demonstrated, by an analysis of classic examples, that the modal content of supposed a posteriori necessities is more complicated than the Kripkean line suggests. We will see that further research is needed concerning the a priori principles underlying all a posteriori necessities. In the course of this analysis it will emerge that the modal content of a posteriori necessities can be best described in terms of a Finean conception of modality – by giving essences priority over modality. The upshot of this is that we might be able to establish the necessity of certain supposed a posteriori necessities by a priori means.

It is hard to exaggerate the extent to which Kripke’s work has changed the way we do philosophy. He has been compared to Socrates and touted as “arguably the world’s greatest living philosopher.” 1 The London Review of Books proclaims that “philosophers are busily rewriting all of semantics (and a good deal of epistemology) in Kripkean terms.” Just what are these terms? The central semantic notion is that of rigid designation, by now a household name. Kripke (Kripke 1972 (henceforth NN)) says that a rigid designator is something that designates the same object in every possible world where it designates at all. Kripke himself has been reluctant to give a theoretical account of rigid designation, preferring instead to talk in terms of a ‘better picture’ which comports with intuition but it seems reasonably clear that Kripke intends this to be a semantic property that some words have (like names and natural kind terms) and that others lack (like definite descriptions). This ‘better picture’ has been developed rigorously by Michael Devitt into a theory (Devitt 1974; Devitt 1981a; Devitt and Sterelny 1999). In the meantime attention has shifted from a general discussion of what rigid designation is to the question of whether and how rigid designation works for natural kind terms like ‘gold,’ and ‘tiger’ (Devitt forthcoming).

It has increasingly been recognised that units of biological classification cannot be identified with the units of evolution. After briefly defending the necessity of this distinction I argue, contrary to the prevailing orthodoxy, that species should be treated as the fundamental units of classification and not, therefore, as units of evolution. This perspective fits well with the increasing tendency to reject the search for a monistic basis of classification and embrace a pluralistic and pragmatic account of the species category. It also provides a diagnosis of the paradoxical but popular idea that species are individuals: Species are not individuals, but the units of evolution are.

Kripke's argument for the rigid designation of natural kind terms is fallacious because he does not distinguish natural kinds from second-order functional properties; by clarifying the concepts of natural kind and functional property, we can show that natural kind terms do designate their referents rigidly, but that functional property terms are not rigid designators. My discussions of functional property will also help dispel the worry about the alleged cases of contingent identity with regard to theoretical statements in science. There is no contingent identity even in the form of second-order logic: Property identity is also a necessary identity. The principle of necessary identity rules relentlessly.

0.138% above the neutron and 0.276% above the proton baryon mass a natural mass unit µ can be identified by extrapolating dimensionless Planck units h=c=1 to the System of Units (SI). Similar to quantum measurements that determine h it is only necessary to relate the unit kinetic particle energy to the quantum energy of a photon having a unit wavelength. Connecting both energies and shifting the units, the inverse ratio of length units evolves proportional to the square of velocity units since both are proportional to the energy unit. With this connection the measurement of h becomes an indirect light velocity measurement and measurement of µ and shows that nonzero action and mass quanta corresponds to a finite light velocity c. As already shown, these sequential baryon mass differences (typical mass deficits of strong interaction) including the electron mass can be recovered within measurement error (some ppm) by simple relations obtained from bosonizing a massive Dirac equation.