untary act, involves a real undertaking to tell the truth. This undertaking may or may not contain all the elements of a contract as they are defined by our law, but nevertheless it imports a legal obligation. An intentional breach of it is deceit; an unintentional breach is mistake.
If the foregoing views of deceit and mistake are sound, it follows that the remedy by way of restitution depends upon the possibility of rescission, that is, of undoing the obligation. Now the representation in the case of mistake is mutual, because the mistake is mutual. If, therefore, the defendant alters his position in reliance upon its truth, rescission cannot be had without his consent and the remedy of restitution cannot be applied. This seems to be the true ground for the doctrine of purchase for value as applied by the learned author in his chapter on "Recovery of Money paid upon Mistake."[1] With this explanation, and with such emendations as would be naturally consequent upon it, the principle of restitution is in accord with the conclusions of that chapter.
The cases which have just been discussed have sufficiently indicated the divergence between the conception of justice entertained by the learned author and the conception involved in the principle of restitution. It remains only to show the difference between the identity required by the principle of restitution to exist between the plaintiff's loss and the defendant's gain, and the enrichment required by the doctrine of unjust enrichment. This difference has incidentally been noted in the discussion of the cases of Exall v. Partridge,[2] and Deering v. Winchelsea.[3] In the former, the plaintiff lost the amount which he paid the landlord, and the defendants profited by their rent, which was paid. The doctrine of enrichment could apply therefore, but the principle of restitution could not, because, since the defendants received nothing from the plaintiff, there was nothing for them to restore. Whatever remedy the plaintiff had was by way of damages, and not by way of restitution. In the latter case there is, for the same reason, no restitution possible. It is somewhat difficult, moreover, to ascertain the measure of enrichment. When one surety pays the whole debt, he loses the full amount. The other surety also profits by the same amount, because he is saved from paying the full amount, and the first inference is that the amount of the debt is the amount
