This paper explores the links between scientific practice and precision both in expert networks and popular literature in the second half 18th century Spain. It will be argued that scientific instruments were used and understood in different ways in these two networks, which required opposing strategies for visualizing the degree and goodness of users' dexterities, thereby fostering the emergence of different collective and individual (epistemic) subjects. I will also argue that these subjects' differences and affinities were constructed around three themes: firstly, the degree of precision needed to establish a correlation between data and the world, or, in other words, the degree of fluidity admitted in connecting material and cultural worlds; secondly, the relevance attributed to body and bod(il)y knowledge in producing reliable data and stabilizing expertise; and thirdly, the weight attributed to opinion in leaning towards ephemeral or lasting data. The first part of the paper looks at the epistemological and political confluences which in late 18th century Spain nurtured the emergence of both a culture of precision and a sphere of public opinion, and to the strained relationship that existed between them. The other three sections explore how Spanish people used different sets of practices to construct different images of themselves as supporters of a moral of precision.