Cooper v Phibbs – Case Summary

Cooper v Phibbs

House of Lords (Ireland)

Citations: (1867) LR 2 HL 149.


Due to a series of inheritances and settlements, the claimant was unaware that the had inherited a life tenancy in a fishery. He believed that the fishery was owned absolutely by his cousins. On this basis, he contracted with his cousins to lease the fishery before discovering his existing interest. The claimant sought to have the lease contract set aside.

  1. Did the claimant and his cousins have a valid contract for the hire of the fishery?

The House of Lords held that since all parties had been mistaken as to the proprietary state of the fishery when they made the contract, the contract could be set aside for mutual mistake.

This Case is Authority For…

If B contracts with C to buy, hire or deal with property which both parties believe belongs to C, when it in fact belongs to B, the defence of mistake arises. This is known as ‘res sua‘ – mistake as to title.


The contract in this case was deemed voidable rather than void. This is despite the fact that common law mistake normally renders a contract void ab initio. The Lords’ justification for this discrepancy was that the claimant only had an equitable interest in the fishery, rather than legal property.