The inquiry of this book has revolved around a rather elementary type of case. A, the beneficial legal owner of an asset, transfers (or purportedly transfers) outright beneficial legal ownership in that asset to a transferee (B), whether in a contractual or in a non-contractual setting, but the transaction between A and B turns out to be in some way defective, for example because A labours under some kind of a mistake. In this context, the question has been analysed – at common law and in equity – whether and in how far A is entitled to claim proprietary rights in the transferred original asset itself and/or in the traceable proceeds thereof. In contrast to many of the leading academic accounts in this field, however, we have considered both a potential acquisition of new property rights (proprietary restitution) and a potential retention of pre-existing property rights. A priori excluding the latter from the scope of one's inquiry would render the results seriously incomplete, if not even prejudiced in substance.

This subject matter required consideration of various taxonomical issues in the context of restitution, unjust enrichment, obligation and property. Taking the Birksian event-based classification of rights as a starting point, it is basically concurred that restitution is multi-causal, in the sense that this response may be triggered by multiple events, not only by unjust enrichment, and that restitution may in principle create personal and/or proprietary rights. In theory, even unjust enrichment may produce restitutionary property rights, but it has been argued that the scope of proprietary rights generated by the event of unjust enrichment is actually much more limited than supposed by Birks and others. In particular, resulting trusts are not based on unjust enrichment. These are rather default trusts which arise in response to a “non-event”, namely a passive retention of a pre-existing equitable interest. Neither are powers in rem, which arise in voidable transfers of property, products of unjust enrichment. Whether a proprietary transaction is valid, void or voidable, and whether a power in rem is imposed in response to a certain defect, is a question of that part of the law which governs consensual transfers of property. Such powers are therefore ultimately a product of the parties’ voidable consent that property shall be transferred.