As the jury evolved from its original form – community members selected for their firsthand knowledge of the details of an offence – into a body of citizens chosen specifically for their lack of any such knowledge, witnesses played an increasingly important role in trials as suppliers of facts. Witnesses were, however, restricted to facts: the courtroom division of labour mandated that the jury alone drew inferences. A clear description of the separate roles of juryman and witness can already be found in the seventeenth century: ‘a Witness swears but to what he hath heard or seen, generally or more largely, to what hath fallen under his senses. But a Juryman swears to what he can inferr and conclude from the Testimony.’

Not all witnesses, however, were restricted to direct observation. There were numerous instances when special or ‘skilled’ witnesses were called to inform the court in areas of expertise supposedly beyond the ken of ordinary folk. Alone among witnesses, these men of skill – be they ship surveyors, insurance brokers, physicians, or grammarians – offered opinions on matters of fact relating to the offence in question. Was this a violation of the fundamental tenet that witnesses were not to offer inferences? To answer this objection, some jurists asserted that such opinion ‘really has the flavor of a fact’, that the testimony of skilled witnesses constituted a ‘class of facts about which expert persons alone could have knowledge’.