Three kinds of things in the universe:
1. Web resources -- unambiguously identified by URLs:
- http://www.example.org/foo.html
2. Non-Web resources:
2.1. Physical objects
- Cars, people, houses, etc.
2.2. Abstract concepts
- Sizes, colors, verbs, "love", etc.
- "Creator" (e.g., the creator of a document)
- "Airline reservation"
