What is the current state of the common law when it comes to the interpretation of the doctrine of assumpsit?

But as early as 1760, Lord Mansfield C.J. characteristically attempted to reframe the particularized approaches of the common law into a universal principle in Moses v. Macferlan (1760), 2 Burr. 1005 at pp. 1008-9, 1012, 97 E.R. 676 at pp. 678, 680-1, when he stated: If the defendant be under an obligation, from the ties of natural justice, to refund; the law implies a debt, and gives this action, founded in the equity of the plaintiff's case, as it were upon a contract ("quasi ex contractu," as the Roman law expresses it). This species of assumpsit, ("for money had and received to the plaintiff's use") lies in numberless instances, for money the defendant has received from a third person; which he claims title to, in opposition to the plaintiff's right; and which he had, by law, authority to receive from such third person. • • • • • This kind of equitable action, to recover back money, which ought not in justice to be kept, is very beneficial, and therefore much encouraged. It lies only for money which, ex aequo et bono, the defendant ought to refund: it does not lie for money paid by the plaintiff, which is claimed of him as payable in point of honour and honesty, although it could not have been recovered from him by any course of law ... In one word, the gist of this kind of action is, that the defendant, upon the circumstances of the case, is obliged by the ties of natural justice and equity to refund the money.