If either is not present, there is no agreement and therefore no contract. When a written contract is not signed by either party, it might still be valid if there is other evidence that both sides intended for it to go into effect. That evidence might be letters, memoranda, or even the beginning of performance by one or both parties. If the contract was for the sale of goods and one party delivered and the other accepted, and this contract also says that you would take full responsibility for problems, then most likely it will be enforceable because both of you partly performed the contract, thus showing acceptance by both sides. For more information you can refer this link.