This paper examines the English case, Regina v Adams in which the difference between "scientific reason" and "common sense" was explicitly at stake in the use of DNA evidence. In its decision the Appellate Court reinstated a boundary between "scientific" and "common sense" evidence, arguing that this boundary was necessary to preserve the jury's role as trier of fact. The paper's discussion of the court's work of demarcation addresses the unresolved problems with the place of probability estimates in jury trials.