Per Lord Brightman in Hart v O'Connor  AC 1000, PC:
The original rule at law, and still the rule in Scotland, was that a contract with a person of unsound mind was void, because there could be no consensus ad idem. This was later qualified by a rule that a person could not plead his own unsoundness of mind in order to avoid a contract he had made. This in turn gave way to a further rule that such a plea was permissible if it could be shown that the other contracting party knew of the insanity.
… It seems to their Lordships quite illogical to suppose that the courts of common law would have held that a person of unsound mind, whose affliction was not apparent, was nevertheless free of his bargain if a contractual imbalance could be demonstrated which would have been of no avail to him in equity. Nor do their Lordships see a sufficient foundation in the authorities brought to their attention to support any such proposition …
To sum the matter up, in the opinion of their Lordships, the validity of a contract entered into by lunatic who is ostensibly sane is to be judged by the same standards as a contract by a person of sound mind, and is not voidable by the lunatic or his representatives by reason of 'unfairness' unless such unfairness amounts to equitable fraud which would have enabled the complaining party to avoid the contract even if he had been sane.