The cause is the basis of the existence and non-existence of a thing and its consideration is prior to consideration of the thing. In accordance with Subjective Theory, the will is the substance of the contract and the other elements of a contract are conditions of will’s efficiency. In contrast, according to Objective Theory, object of the contract is the substance of it and the other elements of a contract are the conditions of object’s efficiency. These two Theories are based on reasons which comparison of them could make it possible to choose one of them in Iranian Civil Law. Acceptance of each of these Theories has some effects in relation to definition, title, parties and unity or duality of the contract and resort to Correctness Principle.