14.3 Ontologies and Knowledge Sharing

14.3.3 Top-Level Ontologies

Example 14.15 defined a domain ontology for apartment building that could be used by people who want to write a knowledge base that refers to things that can appear on maps. Each domain ontology implicitly or explicitly assumes a higher-level ontology that it can fit into. The apartment building ontology assumes buildings are defined.

A top-level ontology provides a definition of everything at a very abstract level.

The goal of a top-level ontology is to provide 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 use different ontologies.

Here we present a top-level ontology based on BFO, the Basic Formal Ontology. Figure 14.7 provides a decision tree which can be used to categorize anything into a number of high-level categories.

1: if entity continues to exist through time then
2:      it is a continuant
3:      if it does not depend on another entity for its existence then
4:          it is an independent continuant
5:          if it has matter as a part then
6:               it is a material entity
7:               if it is a single coherent whole then
8:                   it is an object               
9:          else
10:               it is an immaterial entity          
11:      else
12:          it is a dependent continuant
13:          if it is a property then
14:               if it is a property that all objects have  then
15:                   it is a quality
16:               else if it is a something that can be carried out then
17:                   it is a role
18:               else if it is a something that can happen to an object then
19:                   it is a disposition
20:                   if it is a purpose of an object then
21:                        it is a function                                               
22: else
23:      it is an occurrent
24:      if it depends on a continuant then
25:          if it happens over time then
26:               it is a process
27:          else
28:               it is a process boundary          
29:      else if it involves both space and time then
30:          it is a spatio-temporal region
31:      else
32:          it is a temporal region      
Figure 14.7: Categorizing an entity in a top-level ontology

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

Entities are divided into the disjoint classes of continuants and occurrents. A continuant is something that exists at an instant in time and continues to exist through time. Examples include a person, a finger, a country, a smile, the smell of a flower, and an email. When a continuant exists at any time, so do its parts. Continuants maintain their identity through time. An occurrent is something that has temporal parts, for example, a life, infancy, 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 both occurrents.

A continuant is an independent continuant, or a dependent continuant. 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, a flower, 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 ability to laugh, or the inside of your mouth, or the ownership relation between a person and a phone, can only exist in relation to another object or objects. Note that something that is a part of another object is an independent continuant; for example, while a heart cannot exist without a body, it can be detached from the body and still exist. This is different from a smile; you cannot detach a smile from a cat.

An independent continuant is either a material entity or an immaterial entity. A material entity has some matter as a part. Material entities are localized in space and can move in space. Examples of material entities are a person, a football team, Mount Everest, and Hurricane Katrina. Immaterial entities are abstract. Examples of immaterial entities are the first email you sent last Monday, a plan, and an experimental protocol. Note that you need a physical embodiment of an email to receive it (e.g., as text on your smartphone or spoken by a speech synthesizer), but the email is not that physical embodiment; a different physical embodiment could still be the same email.

A material entity that is a single coherent whole is an object. An object 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). A person, a chair, a cake or a computer are all objects. The left leg of a person (if it is still attached to the person), a football team or the equator are not objects. If a robot were asked to find three objects, it would not be expected to bring a chair and claim the back, the seat and the left-front leg are three objects.

A dependent continuant depends on other objects. One type if dependent continuent is a property. The following are subtypes of properties:

  • 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.

  • 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, and the role of a desk to support a computer monitor.

  • A disposition is something that may 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 struck when not wet.

  • A function is a disposition that is a purpose of an object. For example, the function of a cup may be to hold coffee; the function of the heart is to pump blood.

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). Sunday, March 1, 2026, is a temporal interval; 3:31 p.m. EST on that day is a temporal point. Tuesdays from 3:00 to 4:00 GMT 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 process is something that happens over time, has temporal 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 holiday, writing an email, and a robot cleaning the lab are all processes.

  • 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.

Designing a top-level ontology is difficult. It probably will not satisfy everyone. 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.