From the 1870s on, in Germany initially, and then in various other European countries, rational mechanics, as an instance of a complete, closed science, yielding results of unquestionable validity, became a subject for critical debate. Philosophers and scientists questioned the discipline's basic concepts and grounding principles, scrutinised traditional formulations and proffered novel ones, and investigated its relation to the givens of experience. Focusing on France, in the years 1890–1905, the present paper is to be taken as a local study, and a contribution to the examination of that international debate. It brings into view, most notably, a number of authors, participants in the then-ongoing debate, who nowadays are generally forgotten or passed over by scientists and historians alike. Through their contributions, the evolution of mechanics may be followed, in its quest for a new concept of scientific knowledge.