 Since Aristotle often calls attention to the imprecision of ethical theory (see e.g. 1104a1–7), it comes as a surprise to many readers of the Ethics that he begins Book VI with the admission that his earlier statements about the mean need supplementation because they are not yet clear (saphes). In every practical discipline, the expert aims at a mark and uses right reason to avoid the twin extremes of excess and deficiency. But what is this right reason, and by what standard (horos) is it to be determined? Aristotle says that unless we answer that question, we will be none the wiser—just as a student of medicine will have failed to master his subject if he can only say that the right medicines to administer are the ones that are prescribed by medical expertise, but has no standard other than this (1138b18–34).