Upper ontology


In information science, an upper ontology is an ontology that consists of very general terms that are common across all domains. An important function of an upper ontology is to support broad semantic interoperability among a large number of domain-specific ontologies by providing a common starting point for the formulation of definitions. Terms in the domain ontology are ranked under the terms in the upper ontology, e.g., the upper ontology classes are superclasses or supersets of all the classes in the domain ontologies.
A number of upper ontologies have been proposed, each with its own proponents.
Library classification systems predate upper ontology systems. Though library classifications organize and categorize knowledge using general concepts that are the same across all knowledge domains, neither system is a replacement for the other.

Development

Any standard foundational ontology is likely to be contested among different groups, each with its own idea of "what exists". One factor exacerbating the failure to arrive at a common approach has been the lack of open-source applications that would permit the testing of different ontologies in the same computational environment. The differences have thus been debated largely on theoretical grounds, or are merely the result of personal preferences. Foundational ontologies can however be compared on the basis of adoption for the purposes of supporting interoperability across domain ontologies.
No particular upper ontology has yet gained widespread acceptance as a de facto standard. Different organizations have attempted to [|define standards] for specific domains. The 'Process Specification Language' created by the National Institute of Standards and Technology is one example.
Another important factor leading to the absence of wide adoption of any existing upper ontology is the complexity. Some upper ontologies—Cyc is often cited as an example in this regard—are very large, ranging up to thousands of elements, with complex interactions among them and with a complexity similar to that of a human natural language, and the learning process can be even longer than for a natural language because of the unfamiliar format and logical rules. The motivation to overcome this learning barrier is largely absent because of the paucity of publicly accessible examples of use. As a result, those building domain ontologies for local applications tend to create the simplest possible domain-specific ontology, not related to any upper ontology. Such domain ontologies may function adequately for the local purpose, but they are very time-consuming to relate accurately to other domain ontologies.
To solve this problem, some genuinely top level ontologies have been developed, which are deliberately designed to have minimal overlap with any domain ontologies. Examples are Basic Formal Ontology and the DOLCE.

Arguments for the infeasibility of an upper ontology

Historically, many attempts in many societies have been made to impose or define a single set of concepts as more primal, basic, foundational, authoritative, true or rational than all others. A common objection to such attempts points out that humans lack the sort of transcendent perspective — or God's eye view — that would be required to achieve this goal. Humans are bound by language or culture, and so lack the sort of objective perspective from which to observe the whole terrain of concepts and derive any one standard. Thomasson, under the headline "1.5 Skepticism about Category Systems", wrote: "category systems, at least as traditionally presented, seem to presuppose that there is a unique true answer to the question of what categories of entity there are – indeed the discovery of this answer is the goal of most such inquiries into ontological categories.
But actual category systems offered vary so much that even a short survey of past category systems like that above can undermine the belief that such a unique, true and complete system of categories may be found. Given such a diversity of answers to the question of what the ontological categories are, by what criteria could we possibly choose among them to determine which is uniquely correct?"
Another objection is the problem of formulating definitions. Top level ontologies are designed to maximize support for interoperability across a large number of terms. Such ontologies must therefore consist of terms expressing very general concepts, but such concepts are so basic to our understanding that there is no way in which they can be defined, since the very process of definition implies that a less basic concept is defined in terms of concepts that are more basic and so more well understood. Very general concepts can often only be elucidated, for example by means of examples, or paraphrase.
  • There is no self-evident way of dividing the world up into concepts, and certainly no non-controversial one
  • There is no neutral ground that can serve as a means of translating between specialized ontologies
  • Human language itself is already an arbitrary approximation of just one among many possible conceptual maps. To draw any necessary correlation between English words and any number of intellectual concepts, that we might like to represent in our ontologies, is just asking for trouble.
  • Any hierarchical or topological representation of concepts must begin from some ontological, epistemological, linguistic, cultural, and ultimately pragmatic perspective. Such pragmatism does not allow for the exclusion of politics between persons or groups, indeed it requires they be considered as perhaps more basic primitives than any that are represented.
Those who doubt the feasibility of general purpose ontologies are more inclined to ask "what specific purpose do we have in mind for this conceptual map of entities and what practical difference will this ontology make?" This pragmatic philosophical position surrenders all hope of devising the encoded ontology version of "The world is everything that is the case.".
Finally, there are objections similar to those against artificial intelligence. Technically, the complex concept acquisition and the social / linguistic interactions of human beings suggest any axiomatic foundation of "most basic" concepts must be cognitive biological or otherwise difficult to characterize since we don't have axioms for such systems. Ethically, any general-purpose ontology could quickly become an actual tyranny by recruiting adherents into a political program designed to propagate it and its funding means, and possibly defend it by violence. Historically, inconsistent and irrational belief systems have proven capable of commanding obedience to the detriment or harm of persons both inside and outside a society that accepts them. How much more harmful would a consistent rational one be, were it to contain even one or two basic assumptions incompatible with human life?

Arguments for the feasibility of an upper ontology

Many of those who doubt the possibility of developing wide agreement on a common upper ontology fall into one of two traps:
  1. they assert that there is no possibility of universal agreement on any conceptual scheme; but they argue that a practical common ontology does not need to have universal agreement, it only needs a large enough user community to make it profitable for developers to use it as a means to general interoperability, and for third-party developer to develop utilities to make it easier to use; and
  2. they point out that developers of data schemes find different representations congenial for their local purposes; but they do not demonstrate that these different representations are in fact logically inconsistent.
In fact, different representations of assertions about the real world, if they accurately reflect the world, must be logically consistent, even if they focus on different aspects of the same physical object or phenomenon. If any two assertions about the real world are logically inconsistent, one or both must be wrong, and that is a topic for experimental investigation, not for ontological representation. In practice, representations of the real world are created as and known to be approximations to the basic reality, and their use is circumscribed by the limits of error of measurements in any given practical application. Ontologies are entirely capable of representing approximations, and are also capable of representing situations in which different approximations have different utility. Objections based on the different ways people perceive things attack a simplistic, impoverished view of ontology. The objection that there are logically incompatible models of the world is true, but in an upper ontology those different models can be represented as different theories, and the adherents of those theories can use them in preference to other theories, while preserving the logical consistency of the necessary assumptions of the upper ontology. The necessary assumptions provide the logical vocabulary with which to specify the meanings of all of the incompatible models. It has never been demonstrated that incompatible models cannot be properly specified with a common, more basic set of concepts, while there are examples of incompatible theories that can be logically specified with only a few basic concepts.
Many of the objections to upper ontology refer to the problems of life-critical decisions or non-axiomatized problem areas such as law or medicine or politics that are difficult even for humans to understand. Some of these objections do not apply to physical objects or standard abstractions that are defined into existence by human beings and closely controlled by them for mutual good, such as standards for electrical power system connections or the signals used in traffic lights. No single general metaphysics is required to agree that some such standards are desirable. For instance, while time and space can be represented in many ways, some of these are already used in interoperable artifacts like maps or schedules.
Objections to the feasibility of a common upper ontology also do not take into account the possibility of forging agreement on an ontology that contains all of the primitive ontology elements that can be combined to create any number of more specialized concept representations. Adopting this tactic permits effort to be focused on agreement only on a limited number of ontology elements. By agreeing on the meanings of that inventory of basic concepts, it becomes possible to create and then accurately and automatically interpret an infinite number of concept representations as combinations of the basic ontology elements. Any domain ontology or database that uses the elements of such an upper ontology to specify the meanings of its terms will be automatically and accurately interoperable with other ontologies that use the upper ontology, even though they may each separately define a large number of domain elements not defined in other ontologies. In such a case, proper interpretation will require that the logical descriptions of domain-specific elements be transmitted along with any data that is communicated; the data will then be automatically interpretable because the domain element descriptions, based on the upper ontology, will be properly interpretable by any system that can properly use the upper ontology. In effect, elements in different domain ontologies can be *translated* into each other using the common upper ontology. An upper ontology based on such a set of primitive elements can include alternative views, provided that they are logically compatible. Logically incompatible models can be represented as alternative theories, or represented in a specialized extension to the upper ontology. The proper use of alternative theories is a piece of knowledge that can itself be represented in an ontology. Users that develop new domain ontologies and find that there are semantic primitives needed for their domain but missing from the existing common upper ontology can add those new primitives by the accepted procedure, expanding the common upper ontology as necessary.
Most proponents of an upper ontology argue that several good ones may be created with perhaps different emphasis. Very few are actually arguing to discover just one within natural language or even an academic field. Most are simply standardizing some existing communication. Another view advanced is that there is almost total overlap of the different ways that upper ontologies have been formalized, in the sense that different ontologies focus on different aspects of the same entities, but the different views are complementary and not contradictory to each other; as a result, an internally consistent ontology that contains all the views, with means of translating the different views into the other, is feasible. Such an ontology has thus far not been constructed, however, because it would require a large project to develop so as to include all of the alternative views in the separately developed upper ontologies, along with their translations. The main barrier to construction of such an ontology is not the technical issues, but the reluctance of funding agencies to provide the funds for a large enough consortium of developers and users.
Several common arguments against upper ontology can be examined more clearly by separating issues of concept definition, language, and facts. For instance, people have different terms and phrases for the same concept. However, that does not necessarily mean that those people are referring to different concepts. They may simply be using different language or idiom. Formal ontologies typically use linguistic labels to refer to concepts, but the terms that label ontology elements mean no more and no less than what their axioms say they mean. Labels are similar to variable names in software, evocative rather than definitive. The proponents of a common upper ontology point out that the meanings of the elements in an ontology depend only on their logical form, and not on the labels, which are usually chosen merely to make the ontologies more easily usable by their human developers. In fact, the labels for elements in an ontology need not be words — they could be, for example, images of instances of a particular type, or videos of an action that is represented by a particular type. It cannot be emphasized too strongly that words are *not* what are represented in an ontology, but entities in the real world, or abstract entities in the minds of people. Words are not equivalent to ontology elements, but words *label* ontology elements. There can be many words that label a single concept, even in a single language, and there can be many concepts labeled by a single word. Creating the mappings between human language and the elements of an ontology is the province of Natural Language Understanding. But the ontology itself stands independently as a logical and computational structure. For this reason, finding agreement on the structure of an ontology is actually easier than developing a controlled vocabulary, because all different interpretations of a word can be included, each *mapped* to the same word in the different terminologies.
A second argument is that people believe different things, and therefore can't have the same ontology. However, people can assign different truth values to a particular assertion while accepting the validity of certain underlying claims, facts, or ways of expressing an argument with which they disagree. This objection to upper ontologies ignores the fact that a single ontology can represent different belief systems, and also representing them as different belief systems, without taking a position on the validity of either.
Even arguments about the existence of a thing require a certain sharing of a concept, even though its existence in the real world may be disputed. Separating belief from naming and definition also helps to clarify this issue, and show how concepts can be held in common, even in the face of differing beliefs. For instance, wiki as a medium may permit such confusion but disciplined users can apply dispute resolution methods to sort out their conflicts. It is also argued that most people share a common set of "semantic primitives", fundamental concepts, to which they refer when they are trying to explain unfamiliar terms to other people. An ontology that includes representations of those semantic primitives could in such a case be used to create logical descriptions of any term that a person may wish to define logically. That ontology would be one form of upper ontology, serving as a logical "interlingua" that can translate ideas in one terminology to its logical equivalent in another terminology.
Advocates argue that most disagreement about the viability of an upper ontology can be traced to the conflation of ontology, language and knowledge, or too-specialized areas of knowledge: many people, or agents or groups will have areas of their respective internal ontologies that do not overlap. If they can cooperate and share a conceptual map at all, this may be so very useful that it outweighs any disadvantages that accrue from sharing. To the degree it becomes harder to share concepts the deeper one probes, the more valuable such sharing tends to get. If the problem is as basic as opponents of upper ontologies claim, then, it also applies to a group of humans trying to cooperate, who might need machine assistance to communicate easily.
If nothing else, such ontologies are implied by machine translation, used when people cannot practically communicate. Whether "upper" or not, these seem likely to proliferate.