§7. Mutual promises are sufficient considerations to make a contract binding; but they must be made at the same time. Such promises support each other. The promise of one party constitutes a sufficient consideration for a promise by the other party. In case the parties are distant from each other, if the proposition is made in writing and sent by mail, and a letter of acceptance is written and put in the mail, the contract is complete, unless, before mailing the letter of acceptance, a second letter has been received containing a retraction of the proposal. §8. Promises which are wholly gratuitous are void; because, being neither a benefit to the promiser, nor an injury to the promisee, they are not regarded in law as a valuable consideration. Hence, subscriptions to public works and charitable, literary, and religious institutions, if they are merely gratuitous, can not be collected, unless they have operated to induce others to advance money, make engagements, or do other acts to their own injury. §9. As gratuitous promises are void for want of consideration, so merely gratuitous services, as voluntarily assisting to save property from fire, or securing beasts found straying, or paying another's debts without request, afford no consideration upon which payment for their value can be lawfully claimed; there being no promise of compensation. But if a person knowingly permits another to do certain work, as plowing his field, or hoeing his corn, although the work may have been commenced without his order or request, his consent will be regarded in law as an _implied promise_ to pay for the value of the labor, unless the circumstances of the case are such as to forbid the presumption. §10. A consideration must also be _possible_, and in accordance with law, sound policy, and good morals. A contract founded upon an impossible consideration is void. No man can be lawfully bound to do what is not in the power of man to do. But it is otherwise, if the thing to be done is only at the time impossible in fact, but not impossible in its nature. Hence, inability from sickness to fulfill an agreement, or the impossibility of procuring an article of a certain kind or quality which a person has agreed to deliver, would not exempt him from liability in damages for the non-performance of his contract. §11. A contract, the consideration of which is _illegal_ or _immoral_, may be avoided by either party. A man can not be held to an agreement to do acts forbidden by the law of God or by the laws of the state. But if an illegal contract has been executed; in other words, if the wrong has been done, the party in the wrong can not renounce the contract; for the general rule is, that no man can take advantage of his own wrong; and the innocent party alone has the privilege of avoiding the contract. If both parties are guilty, neither can, in ordinary cases, obtain relief on a contract that has been executed.