- Tom Gruber's widely accepted definition of ontology in AI
- Barry Smith's definition of ontology highlighting similarities and differences between ontology in philosopy and AI.
- FAQ on an ontology formalism called Conceptual Structures
- Lecture on Aristotle's Categories which is the first theory of ontology
- Properties entry from Stanford Encyclopedia of Philosophy. Note the relation of ontology and properties
- A brief description of Category from Internet Encyclopedia of Philosophy
- A nice site about the history of western metaphysics
- A definition of ontology discussing the received view of ontology
- Barry Smith's article discussing Husserl's pioneering work on formal ontology