GFO Part I Basic Principles
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13 Functions

We understand a function to be an intentional entity, defined in purely teleological terms by the specification of a goal, requirements and a functional item. Functions are commonly ascribed by means of the has-function relation to entities that, in some context, are the realizations of the goal, execute such realizations or are intended by a reliable agent to do so. Functions are considered to be intentional entities and, hence, they are not objective entities of the world, but agent-dependent entities that primarily belong to the mental and social strata.

The pattern of the specification of a function $F$, called a function structure, is defined as a quadruple $STR(F)=(LABEL(F)$, $REQ(F)$, $GOAL(F)$, $FITEM(F))$, where:

  • $LABEL(F)$ denotes a set of labels of function $F$;
  • $REQ(F)$ denotes the requirements of function $F$;
  • $GOAL(F)$ denotes a goal of $F$;
  • $FITEM(F)$ denotes a functional item of $F$.
Except for the label, these are called the function determinants, and they determine a function. Labels are natural language expressions naming the function. Most commonly, they are phrases in the form ``to do something'', e.g. ``to transport goods''. The requirements of the function set forth all the necessary preconditions that must be met whenever the function will be realized. For example, in the case of the function ``to transport goods from $A$ to $B$'', goods must be present at location $A$. Functions are goal oriented entities – specifying a function requires providing the goal it serves. However, goals are not identified with functions, as in (17). The goal of the function is an arbitrary entity of GFO –- referred to also as a chunk of the reality –- that is intended to be achieved by each realization of the function. In the case of transporting goods, the location of the goods at $B$ is the goal. The goal specifies only the part of the world directly affected (or intended to be affected) by the function realization. In our case, it is the relator of goods being located at $B$. Often a goal is embedded in a wider context, being a complex whole, e.g. a fact, configuration, or situation, called final state. A final state of a function includes the goal plus an environment of the goal, therefore making the goal more comprehensible. Here, it is the relator together with its relata, i.e., goods located in $B$.

Functions are dependent entities, in the sense that a function is always the function of some other entity, executing it. The functional item of the function $F$ indicates the role of entities executing a realization of $F$, such that all restrictions on realizations imposed by the functional item are also stipulated by some goal of $F$. In the case of ``to transport goods'', the functional item would be the role universal ``goods transporter''.

Entities are often evaluated against functions. This is reflected in GFO by the relations of realization and realizer. Intuitively, an individual realization of a function $F$ is an individual entity, in which (and by means of which) the goal of $F$ is achieved in circumstances satisfying the requirements of $F$. Take the example of function $F$ ``to transport goods $G$ from Leipzig to Berlin'', and the individual process of transportation of goods $G$ by plane from Leipzig to Berlin. In brief, we can say that the process starts when the requirements of $F$ are satisfied, and ends by achieving the goal of $F$, which, therefore, is the realization of function $F$.

It is important to understand the difference between a function and a realization, in particular with regard to their specification. To specify a function and its structure one must state what will be achieved; representing a realization usually means specifying how something is achieved. Note that not all functions must be realized by a process, as in the above example. In fact, in GFO we do not interpret functions in terms of processes or behaviors as described in (50). Apart from functions that are typically realized by processes or behaviors, we also consider functions realized by presentials. Consider, for instance, a pepper moth with a dark covering sitting on a dark bark. This situation is the realization of the function of camouflaging a moth.

In every realization we find entities that execute this realization. They may be identified by references to functional items. For example, for the function ``to transport oxygen'', the role ``oxygen transporter'' is the functional item. Now consider an individual transport process, i.e., a realization, involving a single red blood cell. That cell has the role ``oxygen transporter'' within this realization. This fact gives rise to a new entity that mediates between the realization and the cell itself, namely the cell as an ``oxygen transporter'' (cell-qua-oxygen transporter). Such an entity is called the realizer of the function and is considered to be a qua-individual, i.e., an instance of a role universal.

Functions are often ascribed to entities, e.g. the function of oxygen transport is assigned to a process of blood circulation. We assign functions to entities by the has-function relation, whose second argument is a function, and the first is one of the following:

  • an entity that is a realization of the function, e.g., for the function of transporting oxygen, the process of blood circulation;

  • an entity that plays the role of the realizer in a realization of a function, e.g. the red blood cell in the process of blood circulation;

  • an entity intended to be a realization or a realizer of a function.

The third case especially refers to artifacts that often inherit their functions from the designer, who intends for them to realize particular functions. The function ascription of that kind is called intended-has-functions. Note that artifacts are not only understood to be entities playing the role of realizers, as, e.g., a hammer that plays a realizer of the function ``to hammer nails''. Additionally, artifacts may play the role of realizations, e.g. the process of transporting goods, which is a realization of the transport function, may be an artifact as well. This holds true especially with regard to services.

The intended-has-functions have a normative character, which allows for assigning such functions to entities that possess them as malfunctions. In short, the entity that has an intended function $F$, but is neither a realization nor a realizer of $F$, is said to be malfunctioning. The flavors and more detailed specification of malfunctions and of other notions outlined above can be found in (13).

Robert Hoehndorf 2006-10-18


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