270 Ulrich Krohs
identifi ed and classifi ed as dysfunctional (Krohs 2004, 2008b).^17 This allows for the fol-
lowing explication of the concept of function:
A function is a contribution of a type-fi xed component to a capacity of a type-fi xed
system (Krohs 2008b). “Contribution” is to be taken in a dispositional meaning, as in
Cummins (1975). To ascribe a function, it is suffi cient to single out the contribution of a
component to the capacity according to the type fi xation.
This explication of the concept of function reconstructs how biologists discern func-
tionality from dysfunctionality without referring to the evolutionary history of an organism
but merely by reference to its physiology and ontogeny.^18 When identifying modules with
the components the explication refers to, function talk in approaches to functional modu-
larization can be understood in this way. The explication is also applicable to function
ascriptions in the technological realm, so it allows for a comparison of fi ndings about
functional modules in biology and technology.^19
A look at functional modules of engineered systems casts some light on the diffi culties
we envisage with respect to functional modularity of biological networks, and also shows
where the diffi culties originate. With respect to technical artifacts, Pahl and colleagues
(2007: 496) discern two kinds of modules: functional modules and production modules.
Though conceptually different, production modules and functional modules usually coin-
cide. The reason for this is to be found in the rational planning of the design process. Early
on, the desired capacities of the system are specifi ed and broken down into functional
modules. Each single functional module is then designed as a separate production module
(Pahl et al. 2007: 499–508). In the realized modular artifact, a production module, or an
assembly of several such, becomes a structural module. Consequently the structural
modules (henceforth “S-modules”) coincide with the functional modules (“F-modules”).
The only reason for this congruence, however, is that the S-modules are designed as real-
izations of F-modules. Such a rationale of the design process is missing in the biological
case: nobody has designed biological systems to have a 1:1 S-module:F-module map. The
modules have evolved by processes of adaptation, response to constraints, self-organiza-
tion, and so on. Since we are confronted with the empirical fi ndings of distributed func-
tionality and overlapping functional modules anyway, it is unsurprising that F- and
S-modules of biological networks are often found not to coincide. To the contrary, cases
where F- and S-modules coincide require explanation.^20 In such cases one must identify
external causes or internal constraints that “adjust” the system in the direction of such
congruence of S- and F-modules.
As an example for a mismatch of F- and S-modules, consider the citric acid cycle. It is
delineated functionally (see Krohs 2004: 173) and consequently must be taken as a meta-
bolic F-module. It is not at the same time an S-module, for the following reasons: Each
metabolic intermediate of the cycle is also involved in many anabolic and catabolic reac-
tions not belonging to the cycle, the so-called anaplerotic and cataplerotic sequences that
heavily infl uence and help regulate the size of the pools of each of the intermediates