sysop’s Blogs


In the face of the iPad being released this weekend, it is worth considering for a moment successful new types of semantic application that merge rich data, clean, simple user interfaces and the context of daily life.


Systems Engineering (SE) is a vast discipline that includes many sub-disciplines. The application of Semantic Web technologies is a natural next step in the evolution of the Systems Engineering IT frameworks and tools. An OWL ontology called SysMO, created by TopQuadrant, has been presented to INCOSE workgroups. This effort recently received renewed interest at the INCOSE International Workshop on “Model-Based Systems Engineering” at Mesa, Arizona in February, 2010.


 In OWL if you create a class (and say nothing further about its definition), you have a concept that will only ever result in extensional sets.  That is, the only way for an instance to become a member of this class, and therefore the set of things of that type, is if someone asserts membership in the class.  
For instance, in the famous pizza ontology ( and the excellent tutorial that accompanies it: (  there are concepts, such as DeepPanBase, ThinAndCrispyBase and most of the Toppings, that have no formal definition.  Something will be an instance of DeepPanBase if someone asserts that it is.  The set of all DeepPanBases is an extensionally defined set.  This isn’t a critique; every ontology has to have some extensional sets.  We can’t build everything out of each other; there is some ground beneath our feet. 


One of my colleagues called the other day and asked if we still relied on the distinction between intensional and extensional sets (really, intensionally and extensionally defined sets). Yes, even more so now.

An extensional set is one whose members are enumerated. An intensional set is one where individuals gain membership through some sort of rule. The employees in your organization are extensionally defined (e.g., someone puts you in the employee master file and you’re an employee). Cheap hotels within ten miles of the Denver Airport would be an intensionally defined set. (No one is maintaining this specific list, and membership will fluctuate based on rates.)