 There is a different model of necessity for the constructionist, however, which is to hold that the necessity in question is revealed a posteriori by our investigations of the phenomenon in question. Saul Kripke (1980), Hilary Putnam (1975) and others defended a causal theory of reference on which some terms (notably natural kind terms) referred to some sort of stuff or essence underlying the central uses of the term (see  Reference: Causal Theories).  Crucially, however, because the reference relation is external, competent users of a term can be radically mistaken about what the term refers to and still successfully refer. In the case of water, for example, Putnam suggests that “water” picks out the sort of stuff that bears the appropriate causal-historical relation to paradigmatic instances in our own causal history (viz. H2O), and this was true even when we did not know what sort of stuff that was (i.e. before we knew the chemical structure). Kripke, Putnam, and others emphasized that claims such as “water=H2O” express necessary though a posteriori truths.