I n every jurisdiction rules of evidence permit the courts to call expert witnesses. An "expert," under the liberal legal principles that now prevail in most court systems, is a person who, by reason of specialized training or experience, has the ability to provide the court with a useful and illuminating opinion, inference, or observation relating to an issue in the case. Even on a subject where ordinary persons have some knowledge or familiarity, most courts will still allow an expert to testify if the expert has something to add that will help judge or jury. Increasingly, these rules are being used to obtain the testimony of "moral experts" in cases that seem to present peculiarly moral issues for decision.