To litigants and to general observers, lawyers apply known principles to the particular; the problem is simply to ascertain what the law is and then to make sure it is implemented. But ‘what is the law?’ can only to a limited degree be kept separate from a second question, ‘what ought the law to be?’, and when an issue is raised which is in some respects novel, the courts will be forced to, in a sense, ‘make’ law. In England today this possibility is limited, but in the centuries before parliament became a massive legislating institution, the scope for judge-made law was much more considerable. As social and economic development brought to light questions which necessitated new answers, these answers were given more by the courts than the legislature. Thomas Kebell's generation stood at the start of this. The pace of change was quickening, and frequent though parliaments were (and wide though their competence was recognised to be), legislative change in the law was occasional and spasmodic. If the law was to adapt, the lawyers had to adapt it.

Yet although this proposition is true, it is no simple task to trace non-statutory change in law, or even to be sure that the lawyers understood wholly what was at stake. As Professor Milsom has argued, the notion of the substantive element in law was late to develop. Obviously it could have little meaning while trial consisted in the proper presentation to God of the contradictory oaths of rival claimants, through either the ordeal or battle.