204 Yoshinobu Kitamura and Riichiro Mizoguchi
functions. Then we examine the difference between our defi nition of technical functions
and the defi nition of biological functions of Johansson and colleagues (2005).
Next we discuss other viewpoints for representing functions. We think that what func-
tion is being performed by an artifact depends on an agent’s viewpoint (perspective). We
discuss such capturing perspectives and show a variety of functions of technical artifacts
based on them. We form categories of functions other than our device-centered defi nition
and show some ontological distinctions, which are aspects for categorization of function
defi nitions.
12.2 Device-Oriented Defi nition of Functions
This section discusses our defi nition of the function of technical artifacts from the device
viewpoint.
12.2.1 Device as a System Structure
Two fundamental tasks for conceptualizing technical artifacts are determining what is a
primitive in the model and how the primitives form the whole. Here we adopt the device-
centered viewpoint from qualitative reasoning research in artifi cial intelligence (e.g., de
Kleer and Brown 1984). The device-centered ontology has been widely adopted for per-
forming engineering tasks including design, such as the German-style design methodology
(Pahl and Beitz 1996). In the device ontology, a device has ports, through which it is
connected to other devices. A device consists of other devices of smaller grain size, which
form a whole-part hierarchy. A device operates on other things we call “operands,” and
thus it changes their physical states. The operand is something that fl ows through the
device via ports and is affected by the device. Examples of operands include fl uid, energy,
motion, force, and information.
12.2.2 Function and Behavior
Before we give clear defi nitions of functionality, the concept of a “behavior” must be
clarifi ed as a basis of function. A distinction between behavior and function has been noted
in philosophy and in the fi eld of qualitative reasoning differently. Here we defi ne behavior
of a device as the change in the attribute value of an operand from the value at the input
port of a device to the value at the output of the device. For example, the increase in the
temperature of steam as it goes through a super-heater is a behavior of a heater. Such a
description of the behavior of an artifact is constant with respect to the artifacts’ situation
and/or their context. By defi nition, it is also independent of the intentions of the designers
and users. In this sense we say that “behavior is not subjective but objective.”^1
Unlike a behavior, a function is related to a context of use (i.e., teleological) and hence
is context-dependent. A behavior can perform different functions according to contexts of