the requirement that the subject matter of the contract be a legal act or legal goods.

the requirement that both parties possess a sufficient level of mental ability, such as being of a minimum age or being of sound mind.

the requirement that there be something of value that is given in exchange for something else of value (quid pro quo)

the requirement that a contract be put into a specified configuration, such as in writing or that the signatures of the contracting parties be witnessed by a third party.