It could be argued that anything the parties fail to agree expressly should not be part of the contract. That, certainly, was the attitude of the early common law, which was concerned mostly with contracts concerning land and contracts for the sale of goods. In respect of sales of land, the contracts tended to be elaborate and in writing (they must be made in writing now), with the consequence that it was fair to conclude that if the parties hadn’t put a particular term in the contract, they didn’t intend that term to be there. In the case of the sale of goods, most persons bought goods from people they knew and it was usually relatively simple to spot defects in the proffered goods before the sale. (An exception then, as now, was with transport. Many of the earlier cases involved defective horses-nowadays it’s cars!)
However, with the coming of the industrial revolution, contracts of sale in which the buyer hadn’t seen the goods prior to delivery began to become relatively common. Either the courts had to insist that if the buyer wanted protection he had to enter into a formal contract, including all the terms on which the parties wished to rely (such formality is often found in primitive systems but is usually abandoned as both the legal system and the economy begin to develop) or, if the informality of sales of goods was to be preserved without potential injustice to the buyer, implied terms had to be developed. The formal contract solution would have been a clearly retrograde step. Therefore, from the early 19th century, the courts began to imply terms into contracts of sale. The process of implication of terms continued, until most types of contract nowadays have certain terms implied into them.