 In the case of most natural kind names, it may be argued, baptisms are hard to identify or even conjecture. In Putnam’s view, for such words reference is determined by speakers’ causal interaction with portions of matter or biological individuals in their environment: ‘water’, for example, refers to this liquid stuff, stuff that is normally found in our rivers, lakes, etc. The indexical component (this liquid, our rivers) is crucial to reference determination: it wouldn’t do to identify the referent of ‘water’ by way of some description (“liquid, transparent, quenches thirst, boils at 100°C, etc.”), for something might fit the description yet fail to be water, as in Putnam’s (1973, 1975) famous Twin Earth thought experiment (see the entry on  reference).  It might be remarked that, thanks to modern chemistry, we now possess a description that is sure to apply to water and only to water: “being H2O” (Millikan 2005). However, even if our chemistry were badly mistaken (as in principle it could turn out to be) and water were not, in fact, H2O, ‘water’ would still refer to whatever has the same nature as this liquid. Something belongs to the extension of ‘water’ if and only if it is the same substance as this liquid, which we identify—correctly, as we believe—as being H2O.