Identity, Structure, and Causal Representation in Scientific Models - Mechanism and Causality in Biology and Economics - page 50

to make himself and the boat more presentable to Rose Sayer (Katharine Hepburn's
character), he cleans and lubricates the valve so that the pressure is released
automatically without the use of the hammer.
In each case, with or without the sticky valve, the steam engine has a typical
operation. Which is proper depends as much on Charlie's relationship with Rose as
on any fact about the steam engine. Charlie cleaned up is a different man; the steam
engine cleaned up is different engine. But there are important senses in which both
are still the same. In the case of the engine, if not the man, we can represent the
preserved mechanism as one in which there is a parameter (or parameters) that
governs whether the engine is in its clean or dirty state.
We may prefer one state for pragmatic reasons and, therefore, wish to analyze
the workings only within the one state. Here, John Anderson's ( 1938 , p. 128) notion
of a causal field is helpful (see also Mackie 1980 , p. 35; Hoover 2001 , pp. 41-49).
The causal field consists of background conditions that, for analytical or pragmatic
reasons, we would like to set aside in order to focus on some more salient causal
system. We are justified in doing so when, in fact, they do not change or when the
changes are causally irrelevant. In terms of representation within the structural
account, setting aside causes amounts to fixing certain parameters to constant
values. The effect is not unlike Pearl's or Woodward's wiping out of a causal
arrow, though somewhat more delicate. The replacement of a parameter by a
constant amounts to absorbing that part of the causal mechanism into the functional
form that connects the remaining parameters and variables. We might, for instance,
wish to conduct our analysis of the African Queen's steam engine entirely in the
spit-and-polished state, by setting the parameter governing the state of the valve to
clean and holding it there.
Cartwright's toaster can be treated in the same manner. A parameter might
represent the state of the bolt holding the rack to the lever: when it takes the
value tight , the operation of the toaster is “as advertized”; when loose , it is a little
wonky; when missing , it does not pop up the toast at all. While there are purposes
for which only tight matters and for which we can treat the bolt parameter as a
constant with that value, impounding the state of the bolt to the causal field, it would
miss a critical point not to notice that broken mechanisms are mechanisms of the
same type as well-functioning mechanisms or that less refined descriptions of
mechanisms are special cases of more refined descriptions. Recognizing the first
is essential to the repairman; recognizing the second is essential to the design
engineer.
The structural account supplemented with the notion of the causal field provides
a tool through which different models, different perspectives on phenomena, may
be brought into systematic relationship one to another. It also allows us to under-
stand hierarchical relationships among causal systems stressed by Simon ( 1996 )
and Wimsatt ( 2007 ). While the examples so far involve physical mechanisms,
economic examples abound. Cochrane ( 1998 , p. 283) points out in a monetary-
policy system similar to ( 3.13 ) and ( 3.14 ) that is interpreted as the slope of the

aggregate-supply curve and is typically treated by monetary economists as a
parameter; yet a body of economic theory and empirical analysis treats

α

α as a