ABSTRACT
A few concrete examples will clarify matters. If you are lucky, you may end today better off than you started it. You may receive a sum of money or a car. Someone else may discharge a debt of yours, thus freeing you from an obligation you would otherwise be under, or render you a service, such as repairing your roof after a gale (or paying a builder to do it). Again, if you have obtained information of commercial or other interest, you may turn it to your advantage and make a profit from it. And so on. Now, in the majority of such cases, the law has nothing to say about this. Gains, like losses, are a fact of social and commercial life. Respect for institutions such as contracts, gifts and other means of transferring wealth means that, like losses, they must prima facie be allowed to lie where they fall. But, this will not always be so: there may well be good reasons not to leave matters as they are. Suppose your payment to me is for a car which I do not deliver, or imagine that it results from nothing more than a computer error by your bank. In such a case, I cannot keep it and must repay you what I received. Or take another instance. Suppose the payment is a genuine gift from you, but that it represents money you have stolen from X; if so, I must repay X. And, as with payments of money, so with other more indirect benefits. If you pay my debt to some third party as a simple gift to me, nothing more need be said: but if you pay it because you have stood surety for me, I must reimburse you the amount you paid. You may give me valuable know-how as a matter of benevolence: but if, in fact, you transferred it to me in confidence because I was your employee, or because we envisaged a joint venture which did not, in the event, come off, I cannot use it for my personal benefit, and I must normally account to you for any profit I make out of it. Yet again, if you mend my roof because you wrongly think there was a contract between us and I encourage you in that belief, I am likely to have to pay you for doing so.
