Synthese https://doi.org/10.1007/s11229-018-01982-0 Causal explanation and the periodic table Lauren N. Ross1 Received: 3 December 2017 / Accepted: 8 October 2018 © Springer Nature B.V. 2018 Abstract The periodic table represents and organizes all known chemical elements on the basis of their properties. While the importance of this table in chemistry is uncontroversial, the role that it plays in scientific reasoning remains heavily disputed. Many philosophers deny the explanatory role of the table and insist that it is “merely” classificatory (Shapere, in F. Suppe (Ed.) The structure of scientific theories, University of Illinois Press, Illinois, 1977; Scerri in Erkenntnis 47:229–243, 1997). In particular, it has been claimed that the table does not figure in causal explanation because it “does not reveal causal structure” (Woody in Science after the practice turn in the philosophy, history, and social studies of science, Routledge Taylor & Francis Group, New York, 2014). This paper provides an analysis of what it means to say that a scientific figure reveals causal structure and it argues that the modern periodic table does just this. It also clarifies why these “merely” classificatory claims have seemed so compelling–this is because these claims often focus on the earliest periodic tables, which lack the causal structure present in modern versions. Keywords Causation · Explanation · Philosophy of chemistry · Classification · Causal reasoning 1 Introduction To the extent that there is a trademark figure for the field of chemistry it is surely the periodic table. Considered by many to capture the “essence” of this scientific domain, the periodic table represents and organizes all known chemical elements on the basis of their properties (Scerri 2007, p. xiii). While the importance of this table in chemistry is I would like to thank Jim Woodward, Eric Scerri, Julia Bursten, and two anonymous reviewers for helpful feedback on this paper. B Lauren N. Ross [email protected] 1 Department of Logic and Philosophy of Science, University of California, 3151 Social Science Plaza A, 92697-5100 Irvine, USA 123 Synthese uncontroversial, the role that it plays in scientific reasoning remains heavily disputed. Many suggest that this table serves as a helpful tool for classifying chemical substances, while denying that it figures in scientific explanation (Shapere 1977, pp. 534–535). The periodic table is said to resemble classifications such as Linnaean taxonomy and the Dewey decimal system, which organize phenomena on the basis of various features without explaining them (Scerri 1997a, p. 239). More specifically, it has been claimed that the periodic table does not figure in causal explanation because it “does not reveal causal structure” (Woody 2014, p. 143).1 Typically, causal explanations involve explaining some outcome by appealing to its causes. If the periodic table lacks causal information or causal structure, this would indicate its inability to participate in such explanations. These claims about the explanatory nature of the periodic table have been related to various topics in philosophy of science. One related set of topics involves the concept of scientific theories, their role in explanation, and their connection to non-explanatory projects such as prediction and classification. Some of these analyses have adopted a theory-centered view of explanation, where explanations involve derivations from or reductions to particular theories. In this work, the periodic table is often used as a paradigmatic example of a non-explanatory project as it cannot be reduced to or derived from any underlying theory (Shapere 1977; Scerri 1997a; Scerri and Worrall 2001; Woody 2014). In this sense, the periodic table is said to be “devoid of theoretical status” as it does “not seek to explain the facts but merely to classify them” (Scerri 1997a, p. 239). These and other analyses go to great lengths to clarify how the table lacks explanatory power, despite being useful for classification and prediction (Shapere 1977; Scerri 1997a; Scerri and Worrall 2001). In recent work, more diverse accounts of explanation have been acknowledged, beyond these theory-centered formulations.2 However, despite this work, it has still been suggested that the periodic table “seems a rather poor candidate for explanatory status” under any of these various accounts (Woody 2014, p. 143). This paper examines the status of the periodic table with respect to causal expla- nation. It engages with claims that the table lacks “causal structure” (Woody 2014,p. 143) and explanatory power (Scerri 1997a, p. 239). When these claims are examined in the context of chemists’ discussions and use of the periodic table, they raise a number of puzzles. First, chemists claim that the table allows them to explain particular proper- ties of the elements and their periodic nature (Chang 2007, p. 326) (Myers 2003, p. 66) (Weller et al. 2014, p. 273). Second, they often invoke information in the table in ways that suggest it has causal implications. The table contains information about chemical and physical properties of the elements, which chemists state are “explained,” “ratio- nalized,” and “determine[d]” by atomic structure (Weller et al. 2014, p. 271), (Myers 2003, pp. 44, 66, 85). The relationship between atomic structure and chemical prop- erties is said to be “best conceived as one of cause and effect, with atomic structure determining chemical properties” (Strong 1959, p. 344). Relatedly, standard chemistry 1 Even those philosophers who ascribe some explanatory status to the periodic table deny its role in causal explanation (Kitcher 1989, pp. 428–429) (Woody 2014, p. 143). 2 A non-exhaustive list of different accounts of scientific explanation include: causal (Woodward 2003), non-causal (Batterman 2010; Saatsi and Pexton 2018), mechanistic (Bechtel and Richardson 2010), unifi- catory (Kitcher 1989), and deductive-nomological accounts (Hempel 1965). 123 Synthese textbooks maintain that a goal of the field is to “explain periodic variations in atomic radii, electronegativities, charges, and covalent bond types in terms of our theory of the electronic structure of the atom itself” (Wulfsberg 1991, p. 371). These consider- ations lead to a number of questions. First, if the table is “merely” classificatory, why do scientists seem to invoke it in explanations of these phenomena? Second, if the table lacks causal structure, why is it cited in explanations that appear to be causal? Furthermore, what exactly does it mean to say that a figure has or “reveals” causal structure as opposed to capturing some non-causal classification? The central nature of this table in the field of chemistry and the strong tension between philosophical and scientific claims about its explanatory status motivate these questions and the search for satisfying answers. This paper argues that the modern periodic table does reveal causal structure in the sense of containing causal information that figures in explanations in chemistry. I provide an analysis of what it means for a figure to reveal causal structure where this distinguishes the project of causal explanation from mere classification, prediction, and description. The rest of this paper is organized as follows. Section 2 reviews background leading up to the development of the modern periodic table. In Sect. 3, the interventionist account of causation is introduced and used to specify a set of criteria for causal structure. These criteria are used to examine the table in Sects. 3 and 4 where information relevant to the explanandum and explanans are examined. Section 5 clarifies differences between the modern periodic table and systems that are “merely” classificatory, such as Linnaean taxonomy and the Dewey Decimal system. Section 6 concludes. 2 The periodic table: some background The modern periodic table originated with developments in the mid-19th century. Soon after the acceptance of a standardized set of atomic weights published by Cannizzaro from 1858–1860, it was discovered that many chemical and physical properties of the elements are a periodic function of their atomic weight (Moeller, 1980 p. 23) (Scerri 2007, p. 66). Properties such as atomic volume were shown to increase in repeated intervals among elements serially ordered by atomic weight, as shown in Meyer’s graph in Fig. 1 (Meyer 1870). In this figure, the repeating peaks correspond to elements that share the property of large atomic volume (Li, Na, K, Rb, Cs), while the valleys correspond to elements with low atomic volume (B, Al, Co, Rh). Attempts to clearly capture this pattern led to a number of representational formats, including some of the earliest periodic tables. Mendeleev produced some of the most well-known examples of these tables in which he ordered elements of increasing atomic weight in columns from top-to-bottom and left-to right, such that their chemical similarity or “family resemblance” was captured along a horizontal dimension, by rows (in later tables, including the modern one, these similarities are captured vertically) (Mendeleev 1869) (Scerri 2007, p. 125).3 This can be seen in Fig. 2 where, for example, the 3 Meyer and others produced similar tables, with horizontal similarities. Mendeleev had other non-tabular representations of chemical periodicity. 123 Synthese Fig. 1 Periodicity in graphical format (Meyer 1870) 14th row from the top identifies elements (Li, Na, K, Rb, Cs, Tl) with the shared properties of large atomic volume and a valence (or combining power) of 1. The consistent periodic relationship between an element’s properties and atomic weight led Mendeleev to use this “periodic law” to make a number of predictions about the behavior of undiscovered elements of particular atomic weights (Moeller et al. 1980, p. 158). Some of these predictions would be confirmed in later work and they are represented by various question marks in his table.4 While many chemical and physical properties were shown to follow a periodic pattern, ordering the elements on the basis of atomic weight sometimes produced inconsistencies in this pattern.