13.3.2 Top-Level Ontologies

Example 13.16 defines a domain ontology designed to be used by people who want to write a knowledge base that refers to apartment buildings. Each domain ontology implicitly or explicitly assumes a higher-level ontology that it can fit into. There is interest in building a coherent top-level ontology to which other ontologies can refer and into which they can fit. Fitting the domain ontologies into a higher-level ontology should make it easier to allow them to interoperate.

One such ontology is BFO, the Basic Formal Ontology. The categories of BFO are given in Figure 13.7.

independent continuant
object aggregate
fiat part of object
boundary of object
dependent continuant
realizable entity
spatial region
temporal region
connected temporal region
temporal interval
temporal instant
scattered temporal region
spatio-temporal region
connected spatio-temporal region
spatio-temporal interval
spatio-temporal instant
scattered spatio-temporal region
processual entity
process aggregate
processual context
fiat part of process
boundary of process
Figure 13.7: Categories of Basic Formal Ontology (BFO). The indentation shows the subclass relationship. Each category is an immediate subclass of the lowest category above it that is less indented.

At the top is entity. OWL calls the top of the hierarchy thing. Essentially, everything is an entity.

Entities are either continuants or occurrents. A continuant is something existing at an instant in time, such as a person, a country, a smile, the smell of a flower, or an email. Continuants maintain their identity though time. An occurrent is something that has temporal parts such as a life, smiling, the opening of a flower, and sending an email. One way to think about the difference is to consider the entity's parts: a finger is part of a person, but is not part of a life; infancy is part of a life, but is not part of a person. Continuants participate in occurrents. Processes that last through time and events that occur at an instant in time are also both occurrents.

A continuant is an independent continuant, a dependent continuant, or a spatial region. An independent continuant is an entity that can exist by itself or is part of another entity. For example, a person, a face, a pen, the surface of an apple, the equator, a country, and the atmosphere are independent continuants. A dependent continuant only exists by virtue of another entity and is not a part of that entity. For example, a smile, the smell of a flower, or the ability to laugh can only exist in relation to another object. A spatial region is a region in space, for example, the space occupied by a doughnut now, the boundary of a county, or the point in a landscape that has the best view.

An independent continuant can further be subdivided into the following:

  • A site is a shape that is defined by some other continuants. For example, the hole in a donut, a city, someone's mouth, or a room are all sites. Whereas sites may be at a spatial region at every instance, they move with the object that contains them.
  • An object aggregate is made up of other objects, such as a flock of sheep, a football team, or a heap of sand.
  • An object is a self-connected entity that maintains its identity through time even if it gains or loses parts (e.g., a person who loses some hair, a belief, or even a leg, is still the same person). Common objects are cups, people, emails, the theory of relativity, or the knowledge of how to tie shoelaces.
  • A fiat part of an object is part of an object that does not have clear boundaries, such as the dangerous part of a city, a tissue sample, or the secluded part of a beach.
  • The boundary of an object is a lower-dimensional part of some continuant, for example the surface of the Earth, or a cell boundary.

A spatial region is three-dimensional (a volume), two-dimensional (a surface), one-dimensional (a line), or zero-dimensional (a point). These are parts of space that do not depend on other objects to give them identity. They remain static, as opposed to sites and boundaries that move with the objects that define them.

A dependent continuant is a quality or a realizable entity. A quality is something that all objects of a particular type have for all of the time they exist - for example, the mass of a bag of sugar, the shape of a hand, the fragility of a cup, the beauty of a view, the brightness of a light, and the smell of the ocean. Although these can change, the bag of sugar always has a mass and the hand always has a shape. This is contrasted with a realizable entity, where the value does not need to exist and the existence can change though time. A realizable entity is one of the following:

  • A function specifies the purpose of a object. For example, the function of a cup may be to hold coffee; the function of the heart is to pump blood.
  • A role specifies a goal that is not essential to the object's design but can be carried out. Examples of roles include the role of being a judge, the role of delivering coffee, or the role of a desk to support a computer monitor.
  • A disposition is something that can happen to an object, for example, the disposition of a cup to break if dropped, the disposition of vegetables to rot if not refrigerated, and the disposition of matches to light if they are not wet.

The other major category of entities is the occurrent. An occurrent is any of the following:

  • A temporal region is a region of time. A temporal region is either connected (if two points are in the region, so is every point in between) or scattered. Connected temporal regions are either intervals or instants (time points). Tuesday, March 1, 2011, is a temporal interval; 3:31 p.m. on that day is a temporal point. Tuesdays from 3:00 to 4:00 is a scattered temporal region.
  • A spatio-temporal region is a region of multidimensional space-time. Spatio-temporal regions are either scattered or connected. Some examples of spatio-temporal regions are the space occupied by a human life, the border between Canada and the United States in 1812, and the region occupied by the development of a cancer tumor.
  • A processual entity is something that occurs or happens, has temporal parts (as well as, perhaps, spatial parts), and depends on a continuant. For example, Joe's life has parts such as infancy, childhood, adolescence, and adulthood and involves a continuant, Joe. A processual entity is any of the following:
    • A process is something that happens over time and has distinct ends, such as a life, a holiday, or a diagnostic session.
    • A process aggregate is a collection of processes such as the playing of the individuals in a band, or the flying of a set of planes in a day.
    • A fiat part of process is part of a process having no distinct ends, such as the most interesting part of a holiday, or the most serious part of an operation.
    • A processual context is the setting for some other occurrent, for example, relaxation as the setting for rejuvenation, or a surgery as a setting for an infection.
    • A boundary of a process is the instantaneous temporal boundary of a process, such as when a robot starts to clean up the lab, or a birth.

The claim is that this is a useful categorization on which to base other ontologies. Making it explicit how domain ontologies fit into an upper-level ontology promises to facilitate the integration of these ontologies. The integration of ontologies is necessary to allow applications to refer to multiple knowledge bases, each of which may each use different ontologies. Designing a top-level ontology is difficult. It probably will not satisfy everyone who must use one. There always seem to be some problematic cases. In particular, boundary cases are often not well specified. However, using a standard top-level ontology should help in connecting ontologies together.