This article examines the development of instruments of knowledge in natural philosophy in early modern Europe. It argues that organum and habitus are the fitting concepts available to us in discussing the role played by instruments and machines in relation to the theory and practice of early modern natural philosophy. It investigates the notion of instrumentality examining the notion of instrument and practice during this period.

Summary At the beginning of the 1730s René Antoine Ferchault de Réaumur published two long memoirs on a new type of thermometer equipped with a specially calibrated scale — known ever since as the Réaumur scale. It became one of the most common ‘standardized’ thermometers in Europe until the late nineteenth century. What made this thermometer so successful? What was it specifically? I will first argue that the real Réaumur thermometer as an instrument was a fiction, a ghost — an (...) idealized instrument. On paper, it was theoretically flawless. In reality, the standardized Réaumur thermometer was most likely never achieved. This article shows that its success was essentially due to a recontextualisation from theoretical natural philosophy — Réaumur's principle of uniformity — to: 1) the context of artisanal knowledge and practices and 2) the context of making and reporting in the Mémoires de l'Académie royale des sciences actual measurements done in the field (in Paris at the Observatory, in Provincial France, in the Colonies, and in the rest of Europe). Réaumur's thermometer was essentially a theoretical method to which was associated a particular scale. It was the instrument's reification for market consumption and fieldwork that gave this specific type of thermometer materiality and authority. Although most Réaumur thermometers ever made were strikingly different from one another, over time the thermomètre de Réaumur designation became a brand, a seal of approval born from customary artisanal practices and cultural habitudes. (shrink)