Mutual mistake of fact voids agreement where both parties misunderstand an essential factual element, excluding value opinions. When both parties share a mistake as to a fact essential to the subject-matter, the agreement is void; an erroneous opinion as to the thing's value is excluded. Mutual mistake nullifies contracts where the factual error affects the existence or identity of the contracted subject, as illustrated by lost cargo, a dead animal sold as alive, or the sale of a life estate when the life-holder was already dead.