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Some may conjure up a universal 'ontology of everything' when they first encounter UMBEL. This vision is wrong and not the intent of this lightweight subject concept reference structure. But, UMBEL is trying to straddle two different worlds and world views, and that can often lead to misunderstandings and misperceptions.
In various venues UMBEL has been described as a backbone, as a roadmap to related content, as a lightweight ontology or a lightweight reference structure. These terms convey the right vision and purpose. However, from a technical standpoint that enables UMBEL to fulfill these purposes, UMBEL's role is to act as middleware.
In this regard, UMBEL can be seen as residing at the infocline between two different worlds and world views.
The Greek base -cline is often applied to gradual transition layers or changes in gradients or slope. A thermocline, for example, represents the layer between the deep and surface ocean. While there is mixing across this layer, it is slower than within the two parts that it separates. Both parts and the thermocline layer itself have quite different properties and temperatures, even though all are ocean and salty water.
The UMBEL infocline acts in a similar manner. On one side of the UMBEL layer is the Cyc knowledge base, with its self-contained, more-or-less closed world of higher order logics, microtheories regarding thousands of knowledge domains, rich predicates, and coherence. It is venerable, solid and proven, but with its own language and world view. Its purpose is also directed to reasoning and inference, driven from a foundation of (generally not codified outside of Cyc) common sense. It was designed well in advance of the creation of RDF or OWL, indeed in advance of the Internet and Web itself.
On the other side of the UMBEL infocline is the entire Web. This is a chaotic, decentralized, distributed knowledge environment representing untold numbers of world views. The specifications of the semantic Web and its languages and vocabularies have been designed expressly with these differences in mind and the means and structures to link and interrelate them. The Web environment — though not exactly incoherent — is also not in its ground state coherent. Indeed, it is the very purpose of existing semantic Web standards and UMBEL to help provide that coherence.
A key aspect of the Web is its open world assumption, defined in SKOS as:
What this means for the Web is that we must assume that the system's knowledge is incomplete, and that if a statement cannot be inferred from what is explicitly expressed, we still cannot infer it to be false. Adding new information never falsifies a previous conclusion, and most of what we can know about the world will remain unknown. Cyc, on the other hand, can make closed-world assumptions under appropriate conditions.
UMBEL thus acts as a mediator, or middleware, in its role at this infocline interface between these world views.
The central purpose of UMBEL is to provide a context for relating information. Once such a purpose for context is embraced, the natural next question is: And what shall be the basis for this context?
An accompanying article discusses why OpenCyc was chosen over alternatives as this contextual basis. Ultimately, the reasons for choosing Cyc come down to real practical tools and capabilities such as helping to disambiguate the identities of named entities, mapping ontologies and schema, doing natural language processing, and the proven solidity of the concept relationships that are at the core of UMBEL.
We can view the Cyc knowledge base as a complete, albeit large, world unto itself. Like the Earth, it is complex and varied and self-contained. It has its own atmosphere and perspective on the broader universe:
There is 25 years and perhaps close to 1000 person-years of development behind Cyc. It has thousands of able practitioners around the world and has been used in hundreds of meaningful projects and engagements. Since its release in 2002, there have been well in excess of 100,000 downloads of its open-source OpenCyc version.
This legacy and history leads to distinct functional and terminology differences from current semantic Web perspectives. For example, the richness of Cyc predicates does not lead to simple mappings to existing OWL and RDFS properties. The notion of class is different than the closest analog in Cyc, the collection. The concept and treatment of individuals and types is different. The 1000 or so microtheory domains in Cyc are not easily transferred or mapped to OWL constructs. Cyc uses reification aggressively to functionally combine concepts from constituent elements, such as in its representation for apple tree. Higher-order logic is not transferable in all cases to the first-order logic (FOL) of the semantic Web. And so forth . . . .
Perhaps most importantly, however, is that Cyc has been designed, built and extended by professional ontologists and related researchers. This brings a degree of consistency and quality control that Web-broad initiatives can not hope to approach.
But, attempts to bridge from the Cyc world to the open world assumptions of the general Web means the translation is lossy, much like what happens in moving from a 16 million palette of 24-bit colors to something less.
Here are some statistics showing the relative size and scope of the ResearchCyc and OpenCyc versions of Cyc, current as of the last official distributions:
| Category | OpenCyc | ResearchCyc |
| Reifiied Terms (Constants and NARTs) | 263,332 | 303,340 |
| Assertions | 2,040,330 | 2,964,161 |
| Deductions | 323,751 | 1,305,354 |
| Unique Predicates (Properties) | 17 (OWL) | ~16,000 |
| Disk Storage (KBs) | 495,104 | 566,400 |
The sheer size and sophistication of either version is too great for easy comprehension and linkage by standard Web resources. Thus, the UMBEL project set out to determine and derive the most fundamental concepts from within OpenCyc. What was desired was a tractable set of subject concept hub nodes from within OpenCyc. A further design criterion was to maintain a 100% consistency with OpenCyc for this subset of subject concepts in order for UMBEL to preserve linkage into the Cyc knowledge base.
Other documentation discusses in detail the vetting and extraction process applied to Cyc, the result of which is currently the identification of about 20,000 subject concepts. These are schematically illustrated by the yellow dots on our Cyc Earth representation:
Of course, these yellow dots are not really physical locations on a globe. Rather, they represent important hub locations within the virtual Cyc knowledge space.
Once removed from the broader knowledge base, we now have a simple skein of these 20,000 subject concepts and their interrelations. We can show this lightweight structure as a ball of subject concept nodes (in red) connected to one another via their graph edges. We can represent this lightweight skein as follows, which has similarities to a hairnet with the nodes represented by the knots (in red) in the net:
This simplistic wireframe representation has been presented before for all 20,000 UMBEL nodes via the Cytoscape graph visualization software (see figure right; click for larger size; also see the accompanying article on visualizing the UMBEL backbone).
The following table shows that the overall size and
complexity of Cyc has been reduced by 1-2 orders of magnitude through
this cleaning exercise, resulting in a lightweight UMBEL structure
about 5-10% of the original size:
| Category | OpenCyc | ResearchCyc | UMBEL |
| Terms or Concepts | 263,332 | 303,340 | 20,896 |
| Assertions | 2,040,330 | 2,964,161 | 285,700 |
| Deductions | 323,751 | 1,305,354 | |
| Unique Predicates (Properties) | 17 (OWL) | ~16,000 | 18 |
| Disk Storage (KBs) | 495,104 | 566,400 | 14,445 |
Very striking is the predicate reduction, which is both a key source of lossiness and a challenge in maintaining a meaningful OWL and RDFS correspondence with the original Cyc. However, since the purpose of UMBEL is context and not direct reasoning or inference, this reduction is OK given its role.
Metaphorically, we can now re-apply this UMBEL skein over the Cyc knowledge base. We have described this visual metaphor as the hairnet over the basketball, with UMBEL being the hairnet, and Cyc (Earth) the basketball:
Note that the UMBEL skein can act and be used fully independently from the underlying Cyc structure or not.
This UMBEL lightweight skein or wireframe structure now is ready to act as middleware, to play its role as an infocline. Each of UMBEL's 20,000 subject concepts is, in effect, a docking port to which external Web data can attach. Once attached, this data can then be related to other Web data via the subject concept relationships in the UMBEL skein. This docking and attachment mechanism can be visualized as follows (click to enlarge):
If you mentally remove the Earth figure (Cyc) above, the UMBEL skein acts solely as a context reference structure for other Web data through its lightweight SKOS taxonomy structure ( narrowerTransitive and broaderTransitive). These are the internal edge relationships of the wireframe structure with the red nodes above.
Though lightweight, this structure is surprisingly powerful in that it also enables tie-ins with external ontology classes, what we have called 'exploding the domain' and provides a reference context for Web data. Without these docking ports via UMBEL's subject concepts, there is no contextual frame of reference and these Web data bits essentially tumble aimlessly in a dark knowledge space.
But one need not stop at the infocline wireframe layer of UMBEL. Because each subject concept (docking port) has a direct correspondence to Cyc, we can dive more deeply into the Cyc knowledge environment. First through OpenCyc and then (via licensing or other arrangements) into ResearchCyc or the full Cyc, another dimension of tools and capabilities can become available. We now have backup and support to assess mappings and assignments and inferences and reasoning.
