This article argues for the importance of material practices and the agency of objects in forming and validating knowledge in early modern Britain. They played a crucial role in the development of technological and scientific culture, as part of a process of appropriation of knowledge, skills, and methods from artisans. Despite this importance, they have so far been largely overlooked. Moreover, this article demonstrates how the belief that "scientific" observations could be communicated reliably and certified through specific objects was rooted in religious and mythicized practices. It discusses the mid-seventeenth-century case of the provincial physician William Durston. To prove his knowledge was reliable, he submitted the tapes used to measure his patients together with witness accounts to the Royal Society in London. This article adds to current debates around Useful and Reliable Knowledge in Europe and shows how different kinds of knowledge and actors shaped the Western path to "modernity."