Many years ago the author advanced the thesis that quasi-contract, or unjust enrichment or restitution may be better explained by what he called a proprietary theory. The most important differentiation is that from tort. Take the case where D retains things belonging to P, thus committing a tort of conversion or one of detinue. The fact remains that whereas unjust enrichment seeks the recovery of money, tort is more concerned with things. It is a very simple difference which accordingly seems minor; yet it is a most significant difference nonetheless. This strongly monetary feature leads to one or two other peculiarities, attention to which further unravels what is so distinctive about quasi-contractual claims. The major peculiarity concerns the defence of change of position, together with that of estoppel. The relationship of unjust enrichment with contract is quite different from, being somewhat weaker than, its relation with tort.