The tradition of semantic externalism that follows Kripke (1972) and Putnam (1975) is built on the assumption that the folk have essentialist commitments about natural kinds. Externalists commonly take the body of empirical data concerning psychological essentialism as support for this claim. However, recent empirical findings (Malt, 1994; Kalish, 2002) call the psychological theory of essentialism into question. This thesis examines the relevance of these findings to both essentialism and semantic externalism. I argue that these findings suggest that these theories fail to reflect folk beliefs about natural kinds and folk natural kind term usage. This leads me to propose an alternative thesis-- the Ambiguity Thesis-- that is better able to accommodate the existing body of empirical data.