The term ‘sources of law’ can be used in two distinct ways - historically and analytically.

A historical treatment involves looking at where the law at any particular time and place could be said to have come from, as English Common law might be said to have its source in the customs of English people or of English lawyers, British colonial laws to have their main source in the English Common law, or much of modern European private law to have its source in the Roman law ofJustinian.

It needs to be emphasized at the start that the analysis of law in this way leads almost inexorably to (or stems almost inexorably from) a model of law consisting of rules whose validity can be determined by reference to their sources. Even in modern, highly developed societies such a model of law would be contested by some on the basis that moral values or political sentiments play a dominant role in determining the outcome of legal disputes; in societies which are not so highly developed it is hardly meaningful to distinguish between legal and other rules of conduct. So far as Roman law is concerned, we can use such a source-based model without substantial qualification when dealing with the late Republic onwards, but the further before that we go, the more important it is to be aware that the distinction between legal rules and social rules might hardly have been meaningful.

Over the course of the millennium between the beginning of the Republic and the reign ofJustinian there were inevitably very substantial changes in the way in which the law was perceived and operated in Rome. Although there was never any radical break with the past there were two major caesuras, the first occurring around 150 BC and the second around AD 200. In the first period the law was in what might be called a pre-scientific stage, largely based on custom and with very little in the way of distinctively legal sources apart from scattered pieces of legis­lation; it can most usefully be thought of as a prologue to the second period. The three or four centuries between about 150 BC and AD 200 mark the mature period of Roman law, characterized most notably by the work of the jurists; it was an age of legal science, when highly able lawyers used their reason to identify ever more complex legal rules. Finally, after the deaths of Papinian, Paul, and Ulpian, the three great jurists of the late second and early third centuries, imperial power came to dominate all aspects of the law; if the second period was an age of science, the third was an age of authority.

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