Understanding Natural Kinds and Modal Properties

In this lecture, the notion of modal properties extends beyond individual entities to include natural kinds, such as water or gold. A key distinction is made between individual physical entities (like water molecules) and the general category of water itself. The lecture challenges the conventional idea of origin stories for natural kinds, postulating that instead of seeking when a certain water molecule came to exist, one should investigate the essence of the kind 'water.' This essence is not tied to individually created or destroyed molecules but rather to the collective characteristics that define the natural kind.

An essential component of this discussion delves into Kripke's theory of necessity, particularly the differentiation between fundamental and non-fundamental natural kinds. Fundamental natural kinds are derived from the basic laws of nature that exist independently of other entities or explanations. For example, an electron is characterized by its role in the laws of physics; if something fulfills that role in a different possible world, it is an electron in that context. In contrast, non-fundamental natural kinds, which account for the majority of entities in our world, obtain their identity from their physical microstructure, which informs and explains the observable macroscopic properties of that kind.

This distinction is demonstrated using the example of water, identified specifically as H2O. The macroscopic properties of water, such as its transparency, boiling point, and density, are closely tied to its chemical structure. The lecture emphasizes that all these characteristics can be explained through an understanding of the microstructure of water as H2O, and thus, the identity of water can be reductively explained by its component molecular structure. An additional concept introduced is that of abductive inference, which is a type of reasoning that seeks the best explanation for observations. Given the strong connection between H2O and the properties of water, this identification is presented as the most ontologically parsimonious hypothesis.

The nature of natural kind concepts is also examined, revealing that they possess an indexical quality. The term ‘water’ doesn’t just refer to a certain range of observable characteristics but is an indexical term that assumes a context where H2O exists. This leads to discussions on modal existence, particularly concerning mythical entities like unicorns, which are stated to never have existed and thus cannot exist in any possible world. This brings forth a distinction between entities that have historical existence (like dinosaurs) and those that are purely fictional (like unicorns). The indexical quality in kind concepts means that one cannot definitively claim the existence of a kind unless they can reference an actual, existent instance.

In conclusion, the lecture emphasizes our cognitive tendency towards folk essentialism—a natural inclination to categorize and conceptualize forms of existence and properties. The nature of our mental concepts, informed by context and reference, illustrates how we understand the identity of natural kinds against possible alternatives. This framework shapes not only philosophical discussions around necessity but also implications for scientific inquiry and understanding of the natural world.