Imagine that you are a judge presiding over the trial of a man named Bill, accused of a grisly murder. The physical evidence is overwhelming, and witnesses have yielded damning testimony. There seems to be no reasonable doubt that Bill committed the murder. Suddenly, the defense asks if it can present images of Bill's brain, produced by magnetic resonance imaging (MRI). Bill's attorneys want to introduce the pictures as evidence that their client has a brain abnormality. They will argue that the abnormality justifies either a verdict of not guilty (because Bill lacked the intent to kill or premeditation to commit murder), or a verdict of not guilty by reason of insanity (because Bill lacked control over his actions), or, at least, a conviction on a lesser offense (because Bill is not fully responsible or possibly just because jurors should feel sorry for people with brain disorders). The prosecution argues that you should not admit the scans, because pictures of Bill's brain and testimony by revered scientists might influence the jury much more than such evidence warrants.