Some agreements may be legally applicable, others may not. 3. Legitimate consideration and purpose: the review or the purpose of an agreement should be legal. It should not be prohibited by law; if so, should not overturn the legislation; should be fraudulent; should not be considered immoral or contrary to public order. If the consideration or object of an agreement is illegal, the agreement would be cancelled. 1) According to the theory of well-being, there is only a reasonable consideration if a promise is made in the benefit of the promise or at the expense of the promise that prompts the promise of something else for the beneficiary of the promise. For example, promises that are not pure gifts are not considered enforceable, as the personal satisfaction that the donor can obtain from the promise by the act of generosity is generally not considered a sufficient inconvenience to obtain adequate consideration. 2) Under the idea of a good deal for exchange, there is appropriate thinking when a promisor makes a promise in exchange for something else. Here is the essential condition that the promisor was given something specifically to induce the promise made. In other words, the theory of good deal for exchange differs from the theory of damage-benefit by the fact that the centre of gravity of the theory of the exchange of parties seems to be the reason for making the promises and subjective mutual consent of the parties, while the emphasis on damage-benefit theory seems to be an objective legal disadvantage or an advantage for the parties.
A tacit contract  is a contract in which the consent of the parties is indicated by their conduct. are acceptable alternative methods for describing a tacit contract; a contract in which the execution of the parties is concluded. The parties agree to a treaty through their actions instead of making a promise. Comfort letters – documents that have been issued in support of an agreement, but have no contractual position. Similarly, a person who is of an unhealthy mind, that is, who is not in a position to make a rational judgment at the time of the agreement, would not agree with an unhealthy person. Thus, an agreement with a person who is generally sane, but occasionally unhealthy minded, will lead to a valid contract if a person was healthy at the time of contract conclusion. The onus of proof that the person was not strong at the time the contract was concluded rests with the applicant. Contract Express is a contract in which the consent of the parties has been expressed orally or in writing in oral or written form. An exchange of promises in which the conditions to which the parties attach themselves are explained orally or in writing at the time of their initiation, or a combination of the two. Whether orally orally, the contract must express a mutual intention to be bound in an understandable manner, and include a certain unconditional offer, acceptance and consideration. Similarly, the agreement in which the consideration or object is fraud is illegal.
While the contract clause never fulfilled its potential to protect a universal right to contractual freedom, American courts began asserting at the end of the 19th century that a contract right without inappropriate state regulation is protected by the 14th Amendment clause.