Along with Kripke, Keith Donnellan, and others, Putnam contributed to what is known as the causal theory of reference. In particular, he maintained in "The Meaning of "Meaning"" that the objects referred to by natural kind terms—such as "tiger", "water", and "tree"—are the principal elements of the meaning of such terms. There is a linguistic division of labor, analogous to Adam Smith's economic division of labor, according to which such terms have their references fixed by the "experts" in the particular field of science to which the terms belong. So, for example, the reference of the term "lion" is fixed by the community of zoologists, the reference of the term "elm tree" is fixed by the community of botanists, and chemists fix the reference of the term "table salt" as sodium chloride. These referents are considered rigid designators in the Kripkean sense and are disseminated outward to the linguistic community.