The default sense of the term “mechanism” pertains to the idea of something compound that can be explained in terms of size, shape, and motion and is made up of parts that together carry out a task or function. From a historical point of view, “mechanism” is one of the crucial concepts that have not only deeply influenced the evolution of scientific disciplines and practice but also shaped the philosophical worldview in various historical contexts. Since the mechanical philosophy is a very complex historiographical category, my main aim is not to provide a fine-grained historical evaluation, but rather to offer a general historical background of this notion. This chapter begins with a discussion of the category of the modern mechanical philosophy (MMP), which is followed by an introductory discussion of the new mechanical philosophy (NMP). I argue that the NMP is presented by the new mechanists as a revision of the MMP, yet they do not provide sufficient justification for their position. Using Kazimierz Ajdukiewicz’s philosophical approach to the notion of worldviews, I analyze the constitutive aspects of the mechanical worldview and its various formulations. Finally, I problematize the historically contingent character of the MMP and the NMP and propose some suggestions on how to compare these two philosophical worldviews.