This address is not about “the rule in Hastings Bass” as Lord Walker reluctantly continued to call it in Futter v Futter and Pitt v Holt. That rule is concerned with trustees who make decisions without having given proper consideration to relevant matters which they ought to have taken into consideration or having taken into account considerations which they ought not to have taken into account.
This address is concerned with causes of action where an essential ingredient is that the claimant has made a mistake. It is essentially concerned with spontaneous mistakes, that is to say mistakes which have not been caused by the culpable conduct of the defendant or others, for example misrepresentation, non-disclosure (where there is a duty to disclose), undue influence, duress, or breach of a fiduciary duty.
Before descending into detail, it is helpful to survey the broad categories of case concerning spontaneous mistakes with which this address is concerned. These are (1) transactions which are void for mistake; (2) transactions where the Court can set aside the transaction because it was entered into under a mistake; (3) transactions where the court can alter the written terms of a contract or other document giving effect to the transaction because it mistakenly does not give effect to the intention of one or more of the parties; and (4) transactions where the claimant is entitled to a personal remedy or (possibly and more controversially) a proprietary remedy for mistakenly conferring a benefit on the defendant.
I am not concerned here to address the rectification of the terms of a will to correct a mistake. That turns on statutory provisions. Nor am I concerned with the doctrine of non est factum – where a party who has executed a document under a fundamental misapprehension as to its nature can plead that it was not his or her deed.