The Concept of Non-Contractual Obligations

condictio ob rem
could therefore develop into a contractual right of rescission; 
see 
M Kaser
, Das römische Privatrecht II. Die nachklassischen Entwicklungen (2nd edn 1975) 419, 
421; 
Zimmermann
(fn 4) 857 ff, both with further references. 
42
See 
R Zimmermann
, Bereicherungsrecht in Europa: Eine Einführung, in: id (ed), Grundstrukturen 
eines Europäischen Bereicherungsrechts (2005) 17, 21; for a more detailed analysis, see 
Schlechtriem
(fn 3) vol I, 403 ff, with further references. 
43
For a comprehensive overview, see 
Schlechtriem
(fn 3) vol I, 403 ff with, further references. 
44
Papinian
, D 12, 6, 54; cf also 
Ulpian
, D 4, 4, 16 pr-3: 
condictio indebiti
of a minor against his 
contract partner. 
45
For a more detailed analysis, see 
C Wendehorst
, Die Leistungskondiktionen und ihre 
Binnenstruktur in rechtsvergleichender Perspektive, in: R Zimmermann (ed), Grundstrukturen eines 
Europäischen Bereicherungsrechts (2005) 47–140, 68 ff. 
46
P Birks/C Mitchell
in: P Birks (ed), English Private Law, 2 vols (2000), vol II [15–104] ff;
S 
Meier
, Irrtum und Zweckverfehlung (1999) 253 ff, 272 ff; see also 
SJ Stoljar
, The Doctrine of 
Failure of Consideration (1959) 75 LQR 53–76, 74 ff; 
J Beatson
, The Use and Abuse of Unjust 
Enrichment (1991) 78 ff; for a comparative perspective, see 
Wendehorst
(fn 45) 56 f.
 
47
For a more comprehensive history of the concept of unjustified enrichment in modern law, see 
Zimmermann
(fn 4) 834–901; cf also 
Jansen
(2003) 120 SavZ/Rom 106–192. For French law, see 
Schlechtriem
(fn 3) vol I, 2 ff; 
H Coing
, Europäisches Privatrecht, vol II (1989) 500 f; 
K Zweigert/H 
Kötz
, Introduction to Comparative Law, transl by T Weir (3rd edn 1998) 545 ff. For English law, see 
D Ibbetson
, A Historical Introduction to the Law of Obligations (1999) 265 ff. For a short, 

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9 
principle against unjustified enrichment. To the contrary: the 
condictiones
were 
especially strict actions in that they did not allow for a defence of disenrichment.
48
Accordingly, until the 18th century, the 
condictiones
were normally not based on 
the principle against unjustified enrichment at all; rather they were classified as 
contractual or quasi-contractual actions and often explained on the basis of an 
implied or fictitious consent.
49
In any event, these claims were not included in the 
early natural law theory of 
restitutio
.
 
It was only in the course of the 19th and 20th 
centuries, when the notion of unjustified enrichment as an independent source of 
obligation was finally acknowledged throughout Europe,
50
that these 
condictiones
were re-conceptualised as non-contractual obligations based on the principle against 
unjustified enrichment.
51
Similarly, the modern English law of unjustified 
enrichment has incorporated many actions that were originally of a contractual 
nature.
52
There are few lawyers today who doubt that it was a genuine legal advance 
to overcome the traditional quasi-contractual explanations of the 
condictiones
and 
their English counterparts.
53
This is not only so because the notion of a quasi-
contract is of little explanatory value. More importantly, obligations to return a 
benefit received under a contractual agreement must normally be explained as