What is the legal principle which applies when there is a mistake of fact on a contract?

The legal principle which applies when there is a mistake of fact on a matter of fundamental importance, common to both parties on a contract, was laid down by Lord Westbury in Cooper v. Phibbs (1867), L.R. 2 H.L. 149 at 170, where he says: … if parties contract under a mutual mistake and misapprehension as to their relative and respective rights, the result is, that that agreement is liable to be set aside as having proceeded upon a common mistake.