Open-World Versus Closed-World Assumptions

One interesting difference between the way inference works in logic programming languages such as Prolog^[64] as opposed to in other technologies, such as the RDF stack, is whether they make open-world or closed-world assumptions about the universe. Logic programming languages such as Prolog and most traditional database systems assume a closed world, while RDF technology generally assumes an open world. In a closed world, everything that you haven’t been explicitly told about the universe should be considered false, whereas in an open world, everything you don’t know is arguably more appropriately handled as being undefined (another way of saying “unknown”). The distinction is that reasoners that assume an open world will not rule out interpretations that include facts that are not explicitly stated in a knowledge base, whereas reasoners that assume the closed world of the Prolog programming language or most database systems will rule out facts that are not explicitly stated. Furthermore, in a system that assumes a closed world merging contradictory knowledge would generally trigger an error, while a system assuming an open world may try to make new inferences that somehow reconcile the contradictory information. As you might imagine, open-world systems are quite flexible and can lead to some very interesting conundrums; the potential can become especially pronounced when disparate knowledge bases are merged.