In this paper I try to make as much sense aspossible of, first, the extensive philosophicalliterature concerned with the status of `Wateris H2O' and, second, the implications ofPutnam's invention of Twin Earth, anotherpossible world stipulated to be just like Earth, except that water is XYZ, notH2O.

This paper argues that typical biological species are natural kinds, on a familiar realist understanding of natural kinds—classes of individuals across which certain properties cluster together, in virtue of the causal workings of the world. But the clustering is far from exceptionless. Virtually no properties, or property-combinations, characterize every last member of a typical species—unless they can also appear outside the species. This motivates some to hold that what ties together the members of a species is the ability to interbreed, (...) others that it is common descent. Yet others hold that species are scattered individuals,of which organisms are parts rather than members. But not one of these views absolves us of the need to posit a typical phenotypic profile. Vagueness is here to stay. Some seek to explain the vagueness by saying species are united by “homeostatic property clusters”; but this view collapses into the more familiar realist picture. (shrink)

Machery's case against hybrids rests on a principle that is too strong, even by his own lights. And there are likely important generalizations to be made about hybrids, if they do exist. Moreover, even if there were no important generalizations about concepts themselves, the term picks out an important class of entities and should be retained to help guide inquiry.

There is a perennial philosophical dream of a certain natural order for the natural kinds. The name of this dream is ‘the hierarchy requirement’. According to this postulate, proper natural kinds form a taxonomy which is both unique and traditional. Here I demonstrate that complex scientific objects exist: objects which generate different systems of scientific classification, produce myriad legitimate alternatives amongst the nonetheless still natural kinds, and make the hierarchical dream impossible to realize, except at absurdly great cost. Philosophical hopes (...) for a certain order in nature cannot be fulfilled. Natural kinds crosscut one another, ubiquitously so, and this crosscutting spells the end of the hierarchical dream. (shrink)