"Law of contract is a peculiar branch of substantive law." How is it peculiar ?

Law of contract is a peculiar branch of substantive law because substantive law imposes rights and duties by law, but under law of contract rights and duties are created by parties and the law of contract only gives the situation in which rights and duties are created.
In the word of Anson, it lays down the circumstances in which promise given by promisor is binding on him. Law of contract is a peculiar law it is called due to its peculiar feature of creation. Generally law is created by legislative body or by state but it is contract in which a law is created by the parties of the contract and it is binding on them.