Causal concepts in biology: Distinguishing pathways from mechanisms

For nearly two decades few topics in philosophy of science have received as much attention as mechanistic explanation. One motivation for these accounts is that scientists frequently use the term “mechanism” in their explanations of biological phenomena (Machamer, Darden, and Craver 2000, 2), (Bechtel and Richardson 2010, xvii) (Wimsatt 1976, 671). Biologists, of course, use a variety of causal concepts in their explanations, including concepts like pathways, cascades, triggers, and processes. Despite this variety, mainstream philosophical views interpret all of these concepts with the notion of a mechanism. For example, Robins and Craver (2009) state that although scientists appeal to terms like “cascades, pathways, systems, and substrates...[w]e use the term mechanism for all of these” (Robins and Craver 2009, 42). Similarly, Craver claims that while scientists “say that they discover systems and pathways in the flow of information, and molecular cascades, mediators, and modulators...[t]he term mechanism could do the same work” (Craver 2007, 3). This mechanistic approach faces a significant problem. Although philosophers use the notion of a mechanism interchangeably with other causal concepts this is not something that scientists always do. Consider the notion of a pathway–examples of this concept include gene expression pathways, metabolic pathways, anatomical pathways, developmental pathways, and ecological pathways, just to name a few. Scientists often distinguish pathways from mechanisms. They claim that a single pathway can be instantiated by different mechanisms, that distinct pathways can have similar mechanisms, and that pathways can be discovered without any knowledge of the mechanisms that underlie them. When introducing scientific material they often promise to discuss both the mechanisms and pathways relevant to the domain of inquiry and they seem to reliably use these concepts in some situations, while not in others.

These points raise a number of puzzles for the dominant mechanistic program. If all or most of the causal concepts in biology are well interpreted with the notion of a mechanism, why do scientists often distinguish mechanisms from these other concepts? Why do they use a variety of causal terms if the single notion of mechanism would suffice? Finally, what explains their seemingly consistent use of particular causal concepts in some situations, while not in others? These puzzles suggest that it is worth exploring how scientists distinguish various causal concepts in biology and how these distinctions matter for understanding causal explanation in this domain.

This talk examines how the mechanism and pathway concepts are used in biology and how they figure in biological explanation. I argue that these concepts have (i) distinct features, that they are associated with (ii) different strategies of causal investigation, and that they figure in (iii) importantly distinct types of explanation. A main theme running throughout this talk is that scientists refer to biological systems as “mechanisms” and “pathways” when they share features with systems in ordinary life that we associate with these concepts. In this sense, they analogize these systems to ordinary life examples as a strategy for making complicated features of complex biological systems more cognitively accessible. While the presence of such analogies in scientific discourse is not a new observation (Hempel 1965, 434) (Lewis 1986, 220) (Mitchell 2003, 16-17), their relevance to the mechanism concept has been surprisingly unexplored. In this talk, I explore this claim and argue for the three points mentioned above. I discuss three main features of the mechanism concept, which include: their constitutive make-up, the fact that they include significant causal detail, and their focus on the “force,” “action,” and “motion” involved in causal relationships. The pathway concept, on the other hand, is characterized by: a sequence of causal steps, where these steps trace the flow of some entity through a system and abstract from significant causal detail, while emphasizing the “connection” involved in causal relationships. I show how these features clarify distinct causal investigative strategies used in discovering “mechanisms” and “pathways” and how they figure in importantly different types of biological explanation. In particular, I argue that there are some biological phenomena for which pathway information is explanatory and mechanistic information is not.

References:

Bechtel, W. and R. C. Richardson (2010). Discovering Complexity. The MIT Press.

Craver, C. F. (2007). Explaining the Brain. pp. 1–329.

Hempel, C. (1965). Aspects of scientific explanation. And Other Essays in The Philosophy of Science. The Free Press.

Lewis, D. A. (1986). Causal explanation. In Philosophical Papers Volume II. Oxford University Press.

Machamer, P., L. Darden, and C. F. Craver (2000). Thinking About Mechanisms. Philosophy of Science.

Mitchell, S. D. (2003). Biological complexity and integrative pluralism. Cambridge University Press.

Robins, S. K. and C. F. Craver (2009). Biological Clocks: Explaining with Models of Mechanisms. In The Oxford Handbook of Philosophy and Neuroscience. Oxford University Press.

Wimsatt, W. C. (1976). Reductionism, levels of organization, and the mind-body problem. In Brain and consciousness: Scientific and philosophical strategies. Plenum Press.