ELEMENTS OF A VALID CON TRACT. As we have seen in Section 406, the essen tial elements of a valid contract are: i. Agreement, or the mutual and genuine concurrence of the parties in the thing to be done or omitted, usually indicated by offer and acceptance. 2. Consideration, or the induce ment which marks the agreement as one justly enti tled to be an act in the law. 3. Parties with capacities to contract. 4. A subject matter not illegal or opposed to public policy. Where all of these elements are pres ent a valid and enforceable contract is created. The effect of the absence of any one of the four requisites will be noted as we proceed to discuss them in their order.