The early English jury was self-informing and composed of persons supposed to have first-hand knowledge of the events and persons in question. The judge instructed the jury on the law, but was himself almost entirely dependent upon the jury for his knowledge of the case. By stating the evidence in a way that made the result it wanted a necessary conclusion, the medieval jury was able to alter the impact of formal rules of law to conform with prevailing social attitudes.
Green, Thomas A. "The Jury and the English Law of Homicide, 1200-1600." Mich. L. Rev. 74 (1976): 413-99.