Only within the last generation have quasi contractual obligations been commonly so called. They were formerly regarded as a species of contract, and to distinguish them from express contracts and contracts implied in fact, i.e. contracts in which a promise is inferred from conduct, were called contracts implied in law. Since, like contracts proper, they were enforced by means of the action of assumpsit, it is not surprising that in a period when more importance was attached to the forms of legal remedies than to the nature of substantive rights, the essential dissimilarity of the two obligations was not observed. The persistent failure to recognize it, however, has resulted in confusion and error, and in many cases has wrought serious injustice. It cannot be too strongly emphasized, therefore, that quasi contracts are in no sense genuine contracts. The contractor's obligation is one that he has voluntarily assumed. He is bound because he has made a promise or undertaking that the law will enforce. And the only difference between an express contract and a contract implied in fact is that in the former the promise or undertaking is verbal, while in the latter it is an implication of the promisor's conduct. But quasi contractual obligations are imposed without reference to the obligor's assent. He is bound, not because he has promised to make restitution - it may be that he has explicitly refused to promise, - but because he has received a benefit the retention of which would be inequitable.
In view of this generic difference between contractual and quasi contractual obligations the use of the term "contracts implied in law " to describe the latter is particularly misleading. This was recognized by Sir Frederick Pollock and Sir William Anson, who adopted from the Roman law and from works on the science of jurisprudence the term "quasi contracts," denoting obligations bearing "a strong superficial analogy or resemblance" 1 to contracts, but essentially different therefrom. Other writers have suggested the term "constructive contracts," i.e. fictional contracts imagined or constructed by the courts for the purpose of affording a remedy.2 Neither of these designations is as happy as would be one that avoided altogether the use of the word "contracts," but they are less objectionable than the term "contracts implied in law." The former of them - "quasi contracts" - has recently come into general use.3