Modern legal systems generally have a category of obligations known variously as restitution, unjust enrichment, unjustified enrichment or some variant of these. Whichever legal system they are found in, they have a common source in Roman law: the stoic idea that enrichment at the expense of another is unjust, obligations quasi ex contractu and the different forms of non-contractual condictiones. This chapter traces the development of the ideas from Justinian to the modern day, picking up ideas of equity along the way. It focuses on the shifting principles underlying the category, typified by its name, rather than its concrete instantiations, aiming to trace the largely unthinking patterns of borrowing from one legal system to another and from one language to another.

This chapter tells the history of legal fictions from the emergence of the common law in the twelfth century until the abolition of the forms of action in 1852. It begins with an overview of the procedural framework which allowed, and encouraged, fictions. It then considers eleven old fictions: (i) dominus remisit curiam; (ii) vi et armis; (iii) geographical fictions; (iv) bill of Middlesex; (v) the writ of quominus; (vi) benefit of clergy; (vii) pleading the belly; (viii) common recovery; (ix) trover; (x) ejectment; and (xi) quasi-contract. For each fiction, I identify the reason for its existence, its development, its manner of operation and its effect on the law. Importantly, the fictions are classified in a way which explains their survival or extinction. This Effect Classification distinguishes between Jurisdictional, Auxiliary and Essential Fictions. The Effect Classification plays a central role in the Acceptance Test, which is the thesis of the book.