A person with a congenital intellectual impairment who has no understanding at all of the documents they sign may successfully invoke non est factum, rendering the contract void, even though the defence of incapacity would require proof of the other party's knowledge. Where a loan contract is void for non est factum and the borrower did not in substance receive the benefit of the funds, the lender's restitutionary claim is limited to the amount actually received and retained by the borrower. Section 6(2) of the Contracts Review Act 1980 (NSW) requires assessment of whether the person themselves carried on or proposed to carry on the relevant business, not merely whether the loan was for business purposes.
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