This paper discusses a problem in semantic analysis of modern standard Thai. The synchronically polysememic morpheme /cay/ has meanings approximately equivalent to English "heart, mind, spirit" and "breath." A purely descriptive approach to this form would require two separate dictionary entries for /cay/. An examination of the history of /cay/, however, sheds interesting light on the modern sememes. The morpheme is described first using internal reconstruction and then the comparative method. The disparate results of these two diachronic approaches are weighed, a choice proposed, and a Proto-Tai source suggested for the modern reflexes. (Author/DO)