Review of Robert W. Batterman’s A Middle Way: A Non-Fundamental Approach to Many-Body Physics - Robert W. Batterman, A Middle Way: A Non-Fundamental Approach to Many-Body Physics. New York: Oxford University Press (2021), 174 pp. $74 (hardcover).

Abstract

Non-Fundamental Approach to Many-Body Physics is an extraordinarily insightful book, far-reaching in its scope and significance, interdisciplinary in character due to connections made between physics, materials science and engineering, and biology, and groundbreaking in the sense that it reflects on important scientific domains that are mostly absent from current literature. The book presents a hydrodynamic methodology, which Batterman explains is pervasive in science, for studying many-body systems as diverse as gases, fluids, and composite materials such as wood, steel, and bone. Following Batterman, I will call said methodology "the middle-out strategy." Batterman's main thesis is that the middle-out strategy is superior to alternatives, solves an important autonomy problem, and, consequently, implies that certain mesoscale structures (explained below) ought to be considered natural kinds.In what follows, I unpack and flesh out these claims, starting with a discussion of the levels of reality and its representation. Afterwards, I briefly outline the contents of the book's chapters and then identify issues that seem to me to merit further clarification. Last, I outline an application of the middle-out strategy for the realism debate. All citations are from A Middle Way, and use of quotation marks without page numbers signifies terms that arise throughout the book.To start, it is helpful to think about systems and their description at three different scales.There is the macroscopic scale of everyday objects that are considered homogenous and are described "top-down" by "non-fundamental" theories like thermodynamics and fluid mechanics, and laws such as the thermodynamic equations of state, the Navier-Stokes equations (for fluids) and Navier-Cauchy equations (for solids). The continuum description of macroscale objects and systems is an idealization of sorts. For example, a dilute solution of sugar (the solute) in water (the solvent) may be treated as a fictional homogenous blob. Of course, in reality, the watersugar solution is composed of molecules. Thus, there is the microscopic scale of atoms and molecules in which systems are considered heterogenous and are represented "bottom-up" by