Semantics in Depth

This Page goes even deeper into the semantic Rabbit Hole to explain some of the Decisions made in the Spoc-Web Implementation. We try to avoid confusing Details and to convey the broad and important Ideas without sacrificing too much Accuracy. We hope not to violate Correctness though. Please feed back any Errors You find in these Documents.

One of the Goals for the semantic Web is for the Computer to "understand" the Data in the World Wide Web. Among others this means, the Computer can

• collect, filter and present semantic Data
• search for related Information on the Internet
• allows to formulate complex Queries involving multiple free Variables (see SPARQL)
• understand the Context of natural Language Queries
• prove or falsify Propositions
• deduce new Information from Data and Metadata using the Rules of Logic

The Calculus for these Operations is called First-Order- or Predicate-Logic. The Algorithms for this are an intense Research Topic since the first Artificial Intelligence Wave in the early 1980s. Unfortunately their Runtime Performance is often very low and there is a Risk of non terminating Processes. Additionally the näive Handling of Rules and Proposition can allow to construct inconsistent or contradictionary Systems.

Since then Research has identified several Levels of Languages together with their associated Inference Engines/ Algorithms with reasonable Performance. Also, as Experience shows, the predicted Worst Case Runtimes are rarely encountered. Thus the current Reasoning Engines become useful for a growing Number of Applications.

The Distinction between these Languages usually too detailed for the Purposes of Spoc-Web, therefore several closely overlapping Concepts between these Languages, together with their URIs have been merged into unique Conceps in Spoc-Web.

The Logic Languages in this Sections are ordered by "Inferential and Expressive Power". Usually this Power also increases the Complexity and decreases Performance.

• RDF is a simple Language to write Meta-Data about URIs
• RDFS defines Classes, but in such an unprecise way, that Inference can lead to wrong Results
• OWL Ontology Inference Language (the Acronym is dedicated to the Owl Character from A.A.Milnes Book Winnie the Poo)
• OWL 2 extends and details the first OWL Standard into 2 Flavors with 3 Profiles each to balance Expressivity with Performance:
• OWL 2 Flavors: DL and Full
  
  • OWL DL (Direct Logic) distinguishes the Instance and the Class Role of a Class Resource/URI. This avoids Confusion, inifinite Recursion and increases Performance.
  • OWL Full allows the dual Use of the Class and Instance Roles and is therefore undecidable and computationally expensive.
• OWL 2 Profiles: EL, RL and QL
  
  • EL = Expression Logic: allows only existential Quantifications, no universal (All) and no Negation/Inverse
  • QL = Query Language: this Language allows to translate Logic Queries into relational Queries
  • RL = Rules Language: reasoning in this profile is already implemented in many standard Rule Languages
    
    

Spoc-Web is designed to share Data between Databases. This requires a common Language that is established by fundamental RDF Classes and Relations defined by the W3C in the Namespaces rdf, rdfs and owl.

While the W3C uses URIs to denote Things, Spoc-Web uses integer Numbers for Speed and Compactness. To facilitate Integration, most of the fundamental Classes have fixed IDs below 1000. This allows Applications to hard-code them into their Code. These Definitions are included in the initial Database You downloaded together with the Spoc-Web Application.

Among these are: