John McDowell argues that ethical behaviour cannot be grounded in an external set of normative rules. Instead, he proposes to ground ethical behaviour in being a virtuous person. A fully virtuous person is able to identify unambiguously any moral fact she is confronted with. McDowell’s strategy seems to be, prima facie, an attractive one as it escapes some of the most serious problems that beset normative ethics. The concept of a virtuous person runs, however, into its own difficulties. It does not seem to be easily compatible with what we know about human psychology, namely about normal perception and cognition. The aim of the paper is to expose the core of the incompatibility.