208 Yoshinobu Kitamura and Riichiro Mizoguchi
on neither a behavior as a player of the function role nor a device as a performer but a
context of use. From the engineering viewpoint, a behavior and (physical features of) an
artifact are specifi c ways of realizing the required function. A function can be realized by
different behaviors (and artifacts) in different ways. Thus a function should be independent
of its realization. This engineering requirement justifi es our defi nition of functions as being
detached from artifacts. Biological functions, however, can be inherent to organs, as dis-
cussed in section 12.2.6.
Boorse (2002) makes a similar distinction in terms of a “weak function statement”
and a “strong function statement.” The former is “[an artifact] x performs the function
Z in the [goal] G-ing of [a given system] S at t iff at t, the Z-ing of X is a causal contribu-
tion to G.” The latter is such as “the function of X is Z” and “X has the function Z.”
Our defi nition of function is a kind of weak function statement. The strong function state-
ment seems to regard a function as a feature dependent on an artifact. We describe such
a statement as a statement about the capacity to perform a function. The existence of a
function in such a sense is potential and hidden until it is induced by a user. People usually
suppose essential functions (discussed in the next section) as the functions that an artifact
has.
The actual performance of functions in our defi nition is similar to functioning in Johans-
son and colleagues (2005) and Dipert (2006) in the sense that it can be realized in temporal
physical space. In Johansson and colleagues (2005), functionings are “processes, subject
to a division into temporal parts” as a SPAN entity. Dipert (2006) points out that “there
is a simple difference between an object having a function, and an object’s functioning in
some way.... I will call what functioning does its activity.” Functioning in our defi nition,
however, is for a behavior to play a role and then to make a function as a role a full
existence.
12.2.5 Essential Functions and Accidental Functions
A device can perform some behavior(s) and behavior can perform some function(s). At
least one of these functions is intended by a designer of an artifact (we call this an “essen-
tial function”). An engineering artifact is designed and manufactured in order to have a
certain capacity to perform its essential function. Thus the essential function provides
the artifact’s identity. The names of many artifacts are derived from their essential
functions.
On the other hand, a user can use a device differently from the use intended by the
designer. In such a case we recognize that the device performs an incidental function (we
call this an “accidental function”) induced by the use. For example, a screwdriver performs
a screwing function as its essential function. A user can use it for hitting (exerting linear
force on) a nail as an accidental function. Such a kind of usage is the realization of one
of its possible functions based on the device’s capacities. This user-induced performance