It is often said that a mistake in order to justify rescission must relate to the intrinsic nature of the bargain; and, in distinction from this, a mistake in regard to a collateral matter or in regard to some matter which formed merely the inducement to a contract is said to be without effect. It seems a better mode of statement to say that a mistake vitally affecting a fact or facts on the basis of which the parties contracted renders their contract voidable by an injured party. That is where the parties assumed a certain state of facts to exist, and contracted on the faith 'of that assumption, they should be relieved from their bargain if the assumption is erroneous. This is a sound principle of justice, and should be applied without any further question as to whether the mistake is intrinsic or extrinsic, or whether it affects identity or quality. Though there may be no mistake as to the identity of a thing to which a contract relates, the basis of the bargain may so clearly be that the thing in question possessed certain qualities, or would fulfil a certain purpose as to make it inequitable to enforce the bargain if this assumption is not true.23 Where the mistake in question is not such as to prevent the formation of a contract, the principle of justice warranting relief is based on something analogous to failure of consideration. There is a failure or prospective failure of the injured party, if not to receive what he bargained for, at least to receive what the parties supposed the bargain included.
22 See supra, Sec.1490.
23 "A mistake as to a matter of fact, to warrant relief in equity, must be material, and the fact must be such that it animated and controlled the conduct of the party. It must go to the essence of the object in view, and not be merely incidental. The court must be satisfied, that but for the mistake the complainant would not have assumed the obligation from which he seeks to be relieved." Grymes v. Sanders, Admr., 93 U. S. 55, 60, 23 L. Ed. 798.