Causality and Levels of Explanation in Biology
Marta Bertolaso and Marco Buzzoni
The Context-Relative Dimension of Biological Explanations: From Mechanisms to Levels
New mechanism (see, for example, Machamer, Darden and Craver 2000; Craver 2001; Glennan 2002; Craver and Darden 2005; Bechtel 2006; Craver and Tabery 2016) has constituted an important driving force in philosophical reflection about explanations in biological sciences. In addition to this, new criteria for causal attribution have been articulated in order to include multi-level causality. At the same time, much of the following philosophical discussion has often and until recently neglected the intrinsic context-relative dimension of the mechanistic account of biological processes, i.e., interlevel regulatory dynamics. Such a dimension already emerged from the analysis of the notion of “level” provided by the new mechanist philosophy (Craver 2002). Since the very beginning, in fact, the new mechanism introduced the idea of the explanatory relevance of levels in the biological sciences. As Craver clearly stated, both the object of inquiry and the aim of the new mechanist proposal include the evidence that biological explanations always involve some kind of reference to different levels of biological organizations (Craver 2002). Given that Craver started from recognizing that many contemporary neuroscientific theories are multi-level descriptions of mechanisms, the new mechanist program focused mainly on the analysis of how levels could get integrated in a multi-level neural mechanism. Craver, therefore, developed a taxonomy of interlevel experimental strategies that integrates the levels into a multi-level mechanism.
We can recognize three context-relative dimensions in such an approach. The first one is related to the organizational definition of mechanisms in biological sciences: “The working of the mechanism depends crucially upon its organization. It depends upon the order of the activities and on their relative rates and durations. It also depends crucially upon the structures, shapes, sizes, orientations, and locations of the components” (Craver 2002). The second dimension is a linguistic-conceptual one. Craver, in fact, is in need of disambiguating three different kinds of “levels”—levels of mere aggregates, functional levels and mechanistic levels—assuming that “each of these kinds is individuated by a different asymmetrical decomposition relation” (Craver 2002). Finally, there is a heuristic dimension, which is discussed looking at the scientific practice: what scientists are after is “a mechanistic decomposition into mechanistic levels—a decomposition into entities and activities organized in the performance of a higher level role” (Craver 2002).
Craver’s main objective was taxonomic in nature, so that it was really difficult (and probably even irrelevant at that time) to clarify to what extent the structural and descriptive levels are related, both in ontological and epistemological terms. However, this difficulty has led to lose sight of the real possibility of mechanistic accounts to explain interlevel dynamics. Causal accounts are strictly related to the very definition of mechanism: “The entities and activities composing mechanisms are organized; they are organized such that they do something, carry out some task or process, exercise some faculty, perform some function or produce some end product” (Craver 2002; see also Machamer, Darden and Craver 2000), while statements like “There is no consensus about the mechanisms that produce LTP [Long Term Potentiation]” (Craver 2002; see also Machamer, Darden and Craver 2000) leave our understanding of the biological dynamics open to pluralistic accounts. In other words, the fact that the new mechanism assumed an account of mechanism without a discussion of how entities and activities (i.e., the relata in the explanatory account) are identified in scientific practice, may be seen as a weak point in its defence (Tabery 2004).
At that point, the philosophical reflection changed. On one hand, authors like Mitchell (2008), Paslaru (2009), Strand and Oftedal (2009), Potochnik and McGill (2011) and Bitbol (2014) started reflecting on methodological and conceptual questions concerning causal explanation in biology. They all stressed the context dependence of levels, rightly understood in connection with the pragmatic aims and with an interventionist account of causality. There are, however, important differences. In Bitbol, for example, the expression of alternative or multiple levels of explanations is reframed in terms of interlevel explanations for which an “interventionist-constitutive conception of causation” holds. Accordingly, no level of organization can claim any privilege for itself, because every such level is “constituted” by a certain scale of intervention and observation. For Bitbol, who relies heavily on Kant’s theory of knowledge, our causal interventions are “constitutive” of levels, and this is meant to express: “(i) the relativity of any causal scheme to the method of active substitution of antecedent by means of various instruments adapted to various scales or levels, (ii) [. . .] any causal scheme has these methods and instruments as a necessary presupposition, and (iii) the conviction that there is no “fact of the matter” as to which of the many instrument-relative causal schemes is more “real” than another” (Bitbol 2014: 694-695).
A more moderate version of the context-relativity thesis (and closer to the view that will be defended in this paper) moderate is exemplified by Mitchell (2008). She also stresses the context dependence of the notion of explanatory level without losing important methodological devices such as modularity, hierarchies and robustness in biological systems. These devices play an interesting role in clarifying some features of causal knowledge and the possibility we have to export it from a discipline to another one. In Mitchell’s view, in particular, robustness raises challenges for some of our standard notions of causation and may require “explanations to shift causal agency to other levels of organization to preserve features that are both explanatory and exportable”, where “this shift of level is both variable and context sensitive” (Mitchell 2008). The meaning of “level” is, in these cases, linked to the complex biological systems behaviour, which depends on (1) multilevel organization, (2) multicomponent causal interactions, either within or among levels of organization, as well as (3) evolutionary and other forms of contingency (Mitchell 2008).
On the other hand, other authors highlighted how “[t]echnology—where a wealth of data has made it possible to derive insights into the nature and consequences of molecular function across different timescales and multiple organizational levels of biological systems—has changed the question: to understand how any particular biological system functions over time” (O’Malley et al. 2014: 812). In this case, “[L]evels are not just nested spatial hierarchies but encompass heterogeneous clusters of causal properties related on different time scales that are relevant to particular inquiries” (O’Malley et al. 2014: 812). The causal story goes back in this way to the original question about the interlevel regulatory nature of biological dynamics, which clearly did not find a completely satisfactory account within the new mechanism. The peculiar dynamics that the mechanistic account always tried to grasp led the new mechanism to include in its explanatory account criteria of interlevel causal attributions without having the epistemological tool to account for this integration.
Given that a relativized view of levels in biological explanations is common to these authors, in the following sections we aim to show how the context-dependence differently addressed not only by the mechanist philosophers but also by the aforementioned authors might merge in scientific practice with a wider notion of level. In particular, in the next section, we shall briefly discuss how an agency theory of causation actually meets the challenge of including context dependence in the mechanistic account, proposing a pluralistic, but not relativistic view of causal attributions and scientific explanations. Moving from an agency theory of causality, the main aim of this paper is to clarify how different explanatory levels correspond to different pragmatic interests and practical possibilities, and how this does not exclude an objective character of the notions of mechanism, level and component in specific explanatory contexts. For this purpose, it will be convenient to discuss Eronen’s recent criticism of the mechanist concepts of levels, mechanisms and components. As we shall see, this objection does not present a difficulty for Craver and many other mechanist philosophers only if, and as long as, they consistently hold a context-dependent, but not relativistic, notion of mechanism, level and component (Section 3). Finally, in Section 4, a case study is developed, which purports to illustrate how the discussed context dependence works in the scientific practice.1