In the tradition of essentialism, essences are the underlying structures of reality. According to modern essentialists, they are the hidden causes of things as they appear to us; they give rise to the nature of things qua members of a kind, distinguishing the members of one kind from other kinds (see, for example, Ellis 2001).1 According to this approach, the essence of a natural kind is the property (or subset of properties) which gives rise to the other intrinsic, nonaccidental properties of its members, including their causal powers and dispositions in interactions with their environment. Some versions of essentialism contend that these essences are the most fundamental constituents which exist at a basic physical level, such as the level of atomic and subatomic particles. For example, water is essentially H2O. Its chemical structure is causally responsible for all of the non-accidental properties and interactions of water, such as color, taste, smell, its freezing temperature under certain conditions, etc.