 In the final part of this section about structure, we consider a favorite example of philosophers: the thesis that “Water is H2O.” This thesis is often taken to be uncontroversially true and is used as evidence for semantic externalism and for essentialism about natural kinds (Kripke 1980; Putnam 1975,  1990). Since general theses about the theory of reference and semantic externalism are beyond the scope of this article, we focus narrowly on chemical essentialism. Is having a common essential microstructure sufficient to individuate chemical kinds and explain their general features? And if so, is “being H2O” sufficient to individuate water?