Introduction to ontology semantics and reasoning

I recently had the pleasure to present at the OntoSpot meeting at EBI to help my colleagues gain an intuitive understanding of ontology semantics and reasoning. In this talk I assume that you have a very basic understanding of what an ontology is, but I assume no previous knowledge wrt logic. I provide a number of examples and graphics to explain logic and description logic (DL) concepts.

Here I provide both the slides of this presentation and the link to the recording. If you have any questions or suggestions, please let me know in the comments. I have already had the very helpful suggestion for adding a reference of DL symbols, which I will do shortly.


  1. In the section on speaking about propositional logic, I accidentally said predicate logic instead of propositional logic.
  2. At the end while answering questions, I said RFD rather than RDF.

This video will also be made available at the OBO Academy.

5 thoughts on “Introduction to ontology semantics and reasoning

  1. Hello Henriette. Pleased to meet you. I’m a PhD student working in this crazy domain of the Semantic Web. I’ve just recently become experienced enough to start doing some ontology contract / consulting work for a few clients. Our community is so small, I always like to reach out and see if there are any overlaps in research or goals with others, when I find them. I found your site and have been reading your articles on OWL restrictions and reasoning, as these two subjects have always been a grey area for me. Also, something you might want to post for your readers is a full course on the Semantic Web, by Dr. Harald Sack, of the Hasso Plattner Institute in Germany. It’s a really good course.

    Also, there is the site of Michael Bergman (guy who started KBPedia):… his site has so much useful information for Semantic Web people; it is amazing.

    Anyhow, if you want to chat, see my personal resume website, at Feel free to contact me any time. I’m always interested in talking to others with similar interests!

      • Sure! I have two particular questions that have been bugging me lately.

        1) How do you know when to use equivalentTo over subclassOf when asserting restrictions? I see both used interchangeably sometimes. I know equivalentTo is syntactic sugar for A subclassOf B and B subclassOf A, but is there a reason a class would use equivalentTo on a class restriction expression? To me, it feels weird saying that a defined class is a subclass of some class expression, and then saying that the class expression is a subclass of the defined class, by using equivalentTo. A stackoverflow comment led me to believe that equivalentTo restrictions are for definitions (i.e., necessary and sufficient conditions), and subclassOf is only for hierarchies, so I am a bit confused because I have been using subclassOf to define restrictions for a while.

        2) This may sound stupid, but I want to verify… in Protege, on some class A, say I place 3 restrictions with subclassOf. Each restriction takes up its own line. In the actual file, it’s three separate OWL:restrictions on class A. Does OWL apply these three restrictions as the intersection of them, i.e., the set of individuals that are of type A must adhere to restriction 1, 2 and 3? Is this the same as saying subclassOf (restriction 1) and (restriction 2) and (restriction 3) in one line in Protege?

        Thank you. I’m going to fully watch this reasoning video now. It’s the last hurdle for me with OWL that I never truly grasped.

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