Understanding Modal Properties and Natural Kinds

In this discussion, the focus is on understanding the modal properties of not only individuals like people but also kinds such as water or gold. This involves exploring how we identify these kinds across different possible worlds and what constitutes their essence. A pivotal point is the distinction between individual molecules, like water molecules, and the kind itself, which questions how we define the essence of a substance like water without invoking an unnecessary origin story. The essence of natural kinds can't be understood through creation narratives; rather, it’s crucial to differentiate how kinds are classified, particularly focusing on fundamental versus non-fundamental natural kinds.

Fundamental versus Non-fundamental Natural Kinds
Fundamental natural kinds derive their identity from the basic laws of nature, which are not explained in terms of other properties. For instance, electrons are identified by their behavior as dictated by physical laws. If we consider different possible worlds, we ascertain the presence of electrons by examining their defined roles in those worlds. If they behave differently, they cannot be considered electrons. In stark contrast, most natural kinds fall under the definition of non-fundamental, meaning they derive their identity from their microstructure—the underlying physical components that account for their macroscopic traits.

Microstructure and Macroscopic Laws
To clarify this concept, let’s dive into the example of water and its chemical formula H₂O. Prior to scientific advances in chemistry, various physical and chemical properties of water were known—its transparency, its boiling and freezing points, and its role in solubility. Despite lacking an understanding of the chemical structure, these macroscopic observations paint a picture of water that can be explained adequately through the microstructural lens: the arrangement of hydrogen and oxygen atoms in H₂O. Thus, chemistry not only uncovers the conditions under which water exists but also reveals why water boasts its unique properties, such as its ability to dissolve polar substances while being immiscible with oils.

Abductive Reasoning and Explanatory Efficiency
The methodology of identifying water with H₂O is supported through abductive reasoning—choosing the hypothesis that best explains the correlation between the micro and macro properties we observe. This reductive hypothesis maintains ontological parsimony by reducing the need for multiple separate entities in our understanding of reality; it generates greater explanatory efficiency, leaving fewer aspects unexplained.

Common Mental Framework and Folk Essentialism
The discussion also draws attention to the cognitive aspects of how we inherently categorize the world around us. Folk essentialism is highlighted, indicating that individuals naturally form concepts of kinds without formal instruction based on observable instances and commonly held properties.

Indexical Nature of Natural Kinds
There is an indexical aspect to our concepts of natural kinds—terms like 'water' or 'cat' possess meanings contextually filled by the instances they represent. This indexicality introduces an element of uncertainty; for example, the term 'cat' in a possible world may refer to different entities, akin to machines or genetically engineered life forms, thus calling into question whether they represent the same essence as our earthly cats. This notion underscores that simply assuming similarity in appearance does not lead to equivalency in categorization.

Consequences of Indexicality
The implications of this indexical element lead to assertions like Kripke’s claim that since unicorns never existed, it is necessary that unicorns do not exist in any possible world. The absence of historical referents for unicorns leads to a complete disconnect in their conceptual existence, contrasting with beings like dinosaurs, which once existed and can be referenced through their remains. Essentially, terminology without corresponding instances lacks meaningful reality, where concepts like unicorns fail to fit within the framework of possible existences.

In conclusion, this exploration into the essence of natural kinds and their modal properties provides profound insights into both the philosophical and cognitive underpinnings of how we perceive existence and identity in the natural world. Understanding the interplay between microstructures and observable properties not only sharpens our grasp of philosophy but also enhances our appreciation of the complexities inherent in the classification of kinds.