No subject is fraught with more difficulties for the pleader than that of estoppel. The problems of "when" and "how" to plead seem never so perplexing as when they arise in connection with this subject. That these problems are not confined to any day or age is evidenced by the reports from the time of Lord COKE down to the latest advance sheets of the present day reporter systems, and the lawyers of no generation have been wholly agreed on their solution. No system of pleading yet established has been free from these questions and with each general change in system they seem to spring up with their usual, if not added, perplexity. Conceding this to be the situation, one who attacks the subject with the avowed purpose and intention of clearing up all of the difficulties in it, at the outset, convicts himself of inexperience and temerity. It is with no such expectation that the writer has undertaken this article; he does hope, however, to bring the matter before his readers in such a way by the collection of, and some comment on, the authorities, especially the later ones, as to present and in a small way assist in a solution of, some of the problems as they arise today in pleading cases involving estoppel either as a part of the plaintiff's case or as a defense thereto.
Stoner, W. Gordon. "Pleading Estoppel." Mich. L. Rev. 9 (1911): 484–98, 576–87.