Recent work in cognitive psychology and experimental semantics indicates that people do not categorize natural kinds solely by virtue of their purported scientific essence. Two attempts have been made to explain away the data by appealing to the idea that participants in these studies are reasoning diagnostically. I will argue that an appeal to diagnostic reasoning will likely not help to explain away the data.

Recent experimental studies have claimed to find evidence for the view that natural kind terms such as “water” are ambiguous: that they have two extensions, one determined by superficial properties, the other by underlying essence. In an online experiment, we presented to 600 participants scenarios describing discoveries of novel samples that differ in deep structure from samples of a familiar kind but are superficially identical, such as a water-like substance that is not composed of H2O. We used three different types (...) of question sets to probe whether the participants considered the sample as a member of the kind or not. Our results did not confirm the predictions of the ambiguity view. They were, rather, consistent with views that take underlying essences to be the sole criterion for membership in a natural kind. (shrink)