For the formation of a valid contract it is also necessary that the parties to an agreement must agree for a lawful object. The object for which the agreement has been entered into must not be fraudulent or illegal or immoral or opposed to public policy or must not imply injury to the person or the other of the reasons mentioned above the agreement is void. Thus, when a landlord knowingly lets a house to a prostitute to carry on prostitution, he cannot recover the rent through a court of law or a contract for committing a murder is a void contract and unenforceable by law.