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The flow of substance: a reply to Horsting & Hartjes

W e are pleased that our essay on process thinking in contemporary science (Jaeger & Monk, 2015) still elicits reflection and constructive discussion. We thank Horsting & Hartjes (2022) for their thoughtful engagement with this fundamental philosophical issue, which has lost none of its relevance since the publication of our essay. Horsting & Hartjes (2022) take issue with our definition of “substance” and our claim that process-based explanations are more fundamental than substance-based ones. We would like to clarify three specific points in reply. First, there is an important reason we use the original philosophical definition of “substance” as that which is “universally and eternally unchangeable” as it reflects the perennial search for simple universal principles that underlie the confusing richness and transient nature of our world. The classic example of such substances is the atoms of Greek philosophy: in the atomist view, processes are mere epiphenomena, more or less fleeting rearrangements of unchangeable atoms. Our essay criticises the echoes of this notion, which still reverberate in contemporary science. Horsting & Hartjes, in contrast, use a more modern definition of “substance” as “material or matter, with definite chemical composition and distinct properties.” Substance defined in this way is not useful as a contradistinction to process. Indeed, it embodies the approach of representing dynamic processes in terms of interactions of relatively autonomous and stable material components. Importantly, however, such components can only be defined in terms of the processes from which they are abstracted: the concepts of “composition” and “properties” themselves are fundamentally relational (e.g. “mass” and “charge” refer to interaction propensities). Furthermore, such components are never truly immutable and must ultimately themselves be viewed as processes. It was never our aim to contrast such physical matter with process; in fact, we argue that it is process. Instead, we compare two different modes of explaining phenomena in the material world, arguing that a process perspective leads to better understanding. Second, we do not say that explanation in terms of process should ignore components and their interactions. What we are saying concretely is that to understand thunderstorms, diseases, thought processes and the like, it is not enough to identify their component parts. Material components are interchangeable and are not what define these processes. We, therefore, advocate a shift of focus from the necessary step of identifying components (composition) to the dynamic interactions between them (activity). This does not overrule or replace the analysis of systems through decomposition into material parts but complements it by a dynamic recomposing of the activity that arises from the concerted interaction of these parts (see DiFrisco & Jaeger, 2019). It is not either components or activities, but both components and activities, that are understood as a (limited but useful) representation of process. In this sense, we agree with Horsting & Hartjes (2022) that the two aspects of a system deserve attention. Third, this does, however, not mean that “substances and processes deserve equal credit in science” or that “process-based research should not come at the expense of the importance of substance-based research.” We highlight that there is still far too much emphasis on the search for unchangeable fundamental principles (substance). Since scientific funding is a zero-sum game, this does indeed imply that we need less substance-based research and more process-oriented approaches, simply because the latter have been sorely neglected. More fundamentally, substances and processes are not equivalent in terms of their explanatory power. Substances ultimately cannot be defined and cannot be conceived as existing outside the context of a process. In contrast, there can be subject-less processes, whose nature involves material entities, but does not depend on any individual component’s presence (the hurricane is not in the air molecules, but in their collective pattern). In this sense, processes are more fundamental than substances—they convey greater explanatory power, capturing phenomena that substancebased approaches cannot. The same is not true the other way around. While representation in terms of material composition provides a powerful analytical tool for the study of processes, the organised activity of components can only be properly understood in a process-oriented framework. In summary, we agree with Horsting & Hastjes that material aspects matter; to deny this point was never our intention. Instead, our essay addresses the philosophical issue