The Ontological Level: Revisiting 30 Years of Knowledge Representation

I revisit here the motivations and the main proposal of paper I published at the 1994 Wittgenstein Symposium, entitled "The Ontological Level", in the light of the main results achieved in the latest 30 years of Knowledge Representation, since the well known "What's in a link?" paper by Bill Woods. I will argue that, despite the explosion of ontologies, many problems are still there, since there is no general agreement about having ontological distinctions built in the representation language, so that assumptions concerning the basic constructs of representation languages remain implicit in the mind of the knowledge engineer, and difficult to express and to share. I will recap the recent results concerning formal ontological distinctions among unary and binary relations, sketching a basic ontology of meta-level categories representation languages should be aware of, and I will discuss the role of such distinctions in the current practice of knowledge engineering.