The paper deals with an argument reported by Razi (d. 1210) that was used to attempt to refute the immateriality of human nature. This argument is based on an epistemic asymmetry between our self-knowledge and our knowledge of immaterial things. After some preliminary remarks, the paper analyzes the structure of the argument in four steps. From a methodological point of view, the argument is similar to a family of epistemological arguments (notably, the Cartesian argument from doubt) and is vulnerable to (...) the same objection that can be raised against that form of reasoning. The last section points out that the argument can be used indirectly to highlight the weakness in some arguments for the claim that there is something immaterial in human beings. (shrink)