This article deals with an argument reported by Razi (d. 1210) that attempted to undermine the immaterialist position about human nature. After some introductory remarks and explanation of the conceptual background, the article analyses the structure of the argument, with special attention to the idea of soul-switching.’ Some comparisons are made between the argument reported by Razi and a number of arguments from modern and contemporary eras of philosophy. One section is devoted to the critique of the argument and its (...) conceptual basis. This article shows that the argument reported by Razi is a methodological antecedent of a family of contemporary epistemological arguments against substance dualism. It is also shown that discussion of the argument could be useful to highlight a weakness in some, but not all, versions of immaterialism about human nature. (shrink)