This article inquires into the conditions for the spreading and the success of economic ideas. The case under examination is the strange fate to which Roy Harrod's dynamic theory was subject: although some of his results were incorporated in textbooks, he felt compelled to complain of misinterpretations and misplaced emphasis, and reputed his own approach as more fundamental than mainstream dynamic economics. Harrod was trying to lay the rational foundations of a new theory of cycles and growth without notice by his contemporaries of his attempt. Harrod's failure to win recognition provides an interesting subject for historians of thought, as it enables to appreciate that the production of bright ideas and heuristically valid postulates are not sufficient to convince fellow scientists to adopt a system of thought. Other conditions also seem to be necessary: the author must be able to present his ideas as somewhat new yet belonging to some tradition within the discipline, and must be able to convince researchers that the chosen approach represents "the" rational solution to the problem at hand.