It is of the essence of quasi contractual obligation that the retention of the benefit received by the defendant would be unjust. In a large proportion of the reported cases the presence of this element of obligation is the question in dispute; and since the answer to the question generally depends upon the circumstances under which the benefit is received or obtained, it is convenient to classify quasi contractual obligations according to the nature of such circumstances. Adopting this principle of classification, the subject falls into three grand divisions:
1 Vickery v. Ritchie, 1909, 202 Mass. 247; 88 N. E. 835.
Although, as a matter of analysis, the topic probably should be excluded from the field of quasi contract (see post, Sec. Sec. 260, 270), it seems best to consider also:
IV. The action for restitution as an alternative remedy for repudiation or breach of contract, and for tort.