The Defense Attorney's Dilemma

May an ethical lawyer cross-examine an adverse witness who he knows is telling the truth? The classic law school hypothetical involves the nearsighted bank teller who has identified your client as the bank robber. In court, she is wearing thick glasses, but your client, who has confided to you that he was the robber, remembers that the teller was not wearing glasses when she saw him. Can you ask whether she needs glasses? Whether she was wearing them at the time in question? If she answers truthfully, you can argue, in your closing argument to the jury, that the jury should not convict a presumptively innocent defendant on the basis of the eyewitness identification of a nearsighted woman who wasn't wearing her glasses--even though you know her identification was correct? [I consider that line of questioning unethical. The defense attorney, while he represents his client, is also an officer of the court.]

Alan Dershowitz, Letters to a Young Lawyer, 2001