The study of any legal question may take as a starting point a legal system, located in time and place, and examine the evidence of what the law has actually been at particular times. Alternatively a legal study may start with an idea adapted, as Charles Addison said in 1847 of contract law, ‘to all times and races, and all places and circumstances’, and apply the idea to a particular legal system, rejecting what is there found to be incompatible with the idea. Both approaches have had an important place in legal thinking and writing. Their concurrence is to some extent unavoidable, because the assessment of what the law is at the time of assessment, or of what it was at any previous time, is itself a complex process, requiring elements of historical enquiry, of judgment, of synthesis, and of prediction.

It has not been possible, therefore, to maintain a simple dichotomy in legal analysis between description and prescription. However, the concurrence carries the risk of distorting an understanding of the past. There is a danger that the universal idea may be used to excise or to marginalize aspects of the past that do not conform to it, while at the same time implying that the past, conveniently pruned by these means, offers support for the idea. Addison, for instance, found a ‘surprising … uniformity’ between English and Roman law, but if his universal idea is taken as a starting point the coincidence could not be wholly surprising, for evidence of nonconformity with the idea in both legal systems will have been marginalized by its very nonconformity.