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Mechanistic models of population-level phenomena

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This paper is about mechanisms and models, and how they interact. In part, it is a response to recent discussion in philosophy of biology regarding whether natural selection is a mechanism. We suggest that this debate is indicative of a more general problem that occurs when scientists produce mechanistic models of populations and their behaviour. We can make sense of claims that there are mechanisms that drive population-level phenomena such as macroeconomics, natural selection, ecology, and epidemiology. But talk of mechanisms and mechanistic explanation evokes objects with well-defined and localisable parts which interact in discrete ways, while models of populations include parts and interactions that are neither local nor discrete in any actual populations. This apparent tension can be resolved by carefully distinguishing between the properties of a model and those of the system it represents. To this end, we provide an analysis that recognises the flexible relationship between a mechanistic model and its target system. In turn, this reveals a surprising feature of mechanistic representation and explanation: it can occur even when there is a mismatch between the mechanism of the model and that of its target. Our analysis reframes the debate, providing an alternative way to interpret scientists’ “mechanism-talk”, which initially motivated the issue. We suggest that the relevant question is not whether any population-level phenomenon such as natural selection is a mechanism, but whether it can be usefully modelled as though it were a particular type of mechanism.

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  1. The internal controversies are of course important in their own right (see (Tabery 2004)), but we do not think they bear on the points made in this paper.

  2. In this paper we generally use the term “mechanistic” in conjunction with models and explanations. We note that Glennan uses “mechanical” instead, but we take it that these terms are interchangeable.

  3. Meccano is a toy construction system in the same spirit as the better known Lego, but vastly superior in its ability to produce complex working models.

  4. This dovetails nicely with a view in the literature that non-physical models should be considered imagined concrete objects—objects that do not actually exist, but would be concrete if they did exist (Godfrey-Smith 2006 p. 734–735).

  5. We also note in passing that the definition exhibits some of the same potential for slippage we noted earlier. Woodward’s paper is about mechanisms, and he later mentions that this is a notion of mechanisms that is used in certain branches of science. What he has defined above, however, is a model of a mechanism. That aside, we fully agree with his characterisation. But again, we emphasise that it is essential to distinguish this characterisation from an account of mechanisms themselves.

  6. Glennan provides a number of additional criteria that we shall not discuss here for reasons of space. We take it that these other criteria are not strictly necessary for mechanical adequacy, but may nevertheless improve a mechanistic model’s efficacy.

  7. For a more complete description of the model, see (Barr 2000) or the original paper (Phillips 1950).

  8. See (Woodward 2003, p. 127) for a discussion of what kind of possibility he means.

  9. For example, Carl Craver appears to deny that a purely behavioural mapping from model to target is explanatory at all (Craver 2006, p. 358). On closer inspection, however, he does not rule out the idea that behavioural or phenomenal models can explain. Rather, Craver takes himself to be discussing a particular kind of explanation, and he does not think that this kind of explanation is exhaustive: “Perhaps not all explanations are mechanistic” (Craver 2006, p. 367).

  10. We say that this is a “natural” distinction, which is not the same as saying it will always be clear-cut. It is likely that there will be cases where it is not obvious whether an intervention is internal or external, as we suspect the division to be vague at its boundary, as well as this boundary potentially being perspective-dependent. However, there will also certainly be cases where it is obvious which category the intervention falls into, and we think that these will be the majority of cases.

  11. See also the discussion of “autonomy” in (Pearl 2000, p. 344–345).

  12. Although note that Glennan also claims that some mechanisms include only functionally defined parts (Glennan 2005, p. 459).


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The germ of this paper started a long time ago with the help of Patrick Forber and Ben Jeffares. Key ideas in the paper have benefitted from feedback at workshops and conferences, particularly Philosophy of Biology at Dolphin Beach (2006) and the “Modeling the World” conference in Helsinki (2009). We would also like to thank our anonymous referees.

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Correspondence to John Matthewson.

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Matthewson, J., Calcott, B. Mechanistic models of population-level phenomena. Biol Philos 26, 737–756 (2011).

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