Indebitum solutum and unjustified enrichment

If unjustified enrichment was not founded on contract, the question was bound to arise as to how it could be positively classified. In his Institutes Gaius appears to shirk the problem, but in another work of his, the so-called Res cottidtanae, we find an answer of a sort: together with a variety of other obligations that did not appear to fit into the neat "summa divisio" of contract and delict,²⁵ he placed it in a third class of "obligationes...

codes—among them, most notably, the French code civil,^[4311]—still perpetuate this tradition.

We have so far referred to indebitum solutum (condictio indebiti) and unjustified enrichment interchangeably. This is justifiable only as long as we confine our attention to Gaius' and Justinian's Institutes, for both of them do, indeed, deal only with this one form of enrichment liability. But this was merely a kind of pars pro toto treatment, suitable at best for an introductory textbook. The briefest glance into either Code or Digest will show us that the Roman lawyers recognized a variety of other "condictiones". Yet, so influential was the "institu­tional" abridgement, that writers and legislators of later centuries have sometimes exclusively focused their attention on the condictio indebiti. Again, the French code civil provides the most prominent example:

"Celui qui reqoitpar erreur on sciemment ce qui ne hit estpas du s'oblige a le restituer a celui de qui il I'a indument tqu",

it states in art. 1376, thereby adopting, as it generally tended to, the views propounded by Robert Joseph Pothier.^[4312]