 About the time when philosophers of science turned to biology, a new theory of meaning was widely discussed—the causal theory of reference (Kripke 1972; Putnam 1975; see entries on  reference  and  natural kinds).  Proponents of this view turned against earlier theories of reference, which they took to hold that the reference of a token (an instance of a term’s use) is fixed by a description of the referent the speaker has in mind and that represents the term’s meaning. Instead, according to the new theory, a natural kind term, and especially a scientific term, is connected to a kind of entity when it is used for the first time to refer to an instance as belonging to a certain natural kind by an ostensive gesture or an equivalent. Speakers might then use a scientific term to refer to a natural kind, even if they are not able to identify a member of the kind in question or have false beliefs about this kind of entity. They rely on the fact that there are experts in their broader linguistic community who can identify members of the kind in question in the way it was initially introduced, and who stand in a suitable causal relation with the event of the original naming of the kind through a series of communicative links. The initial identification of a kind might have been based on recognizing some salient shared properties (“nominal essence” in Locke’s terms;  Locke on real essences).  However, if the introduction of the term has indeed successfully identified a natural kind, then, eventually, the “real essence” of the kind will be identified, presumably in form of the physical microstructure, which explains the initially recognized properties. The latter still play a role in characterizing a stereotype for non-expert speakers, but based on the underlying causal properties experts can determine kind membership unequivocally in every case (Putnam 1975).