Dave Mccomb’s Blogs


Brilliant short video that frames the semantic web beautifully


 Semantics and Master Data Management — it’s like chocolate and peanut butter.  



 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 (http://www.co-ode.org/ontologies/pizza/2007/02/12/pizza.owl) and the excellent tutorial that accompanies it: http://www.co-ode.org/resources/tutorials/  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 collegaues 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 memebers are enumerated.  An intensional set is one where individuals gain membership through some sort of rule.  The employees in your organization are extensionally defined (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 fluxuate based on rates).