In support of Frans van Lunteren’s project for big-picture history organized around “mediating machines,” these comments stress “mediation” as active agency in the world rather than as mere metaphor, on the view that this active agency underlies the potency of technologies as mediators, both between different domains of knowledge and between theories and things. Similarly important for this power is the diversity of the particular constructions that constitute mediators like “balances” or “engines.” Diversity of meaning and action gives them their cultural reach, from mechanical contrivance to natural process to political ideology. An interesting question remains about how many mediating machines will suffice for the big picture of modernity over four centuries. Statistics, for example, might be a crucial addition. Another question concerns how to characterize the knowledge regime of a mediating machine. Van Lunteren chooses “information” for the computer. He might also have chosen “complexity,” with different import for the character of postmodernity.

Understanding complex physical systems through the use of simulations often takes on a narrative character. That is, scientists using simulations seek an understanding of processes occurring in time by generating them from a dynamic model, thereby producing something like a historical narrative. This paper focuses on simulations of the Diels-Alder reaction, which is widely used in organic chemistry. It calls on several well-known works on historical narrative to draw out the ways in which use of these simulations mirrors aspects of narrative understanding: Gallie for "followability" and "contingency"; Mink for "synoptic judgment"; Ricoeur for "temporal dialectic"; and Hawthorn for a related dialectic of the "actual and the possible". Through these reflections on narrative, the paper aims for a better grasp of the role that temporal development sometimes plays in understanding physical processes and of how considerations of possibility enhance that understanding.

Alexander von Humboldt provides a point of reference for questions that arise when reflecting on the papers in this special issue on "Experiencing the Global Environment," for he aimed to integrate local and global experience and qualitative and quantitative observation in his conceptions of physiognomy and of instruments. What are we to understand by direct experience? How do we draw the limits of our senses, whether in the larger world or internally? Does recent scholarly interest in distributed cognition illuminate the distributed experience of global phenomena obtained through mapping? How do our concepts shape our experience, whether local or global? Finally, do recent trends in the sciences, emphasizing complexity and contingency, tend to make traditional tensions between local and global priorities and between qualitative and quantitative description less relevant? Humboldt would have thought so.