Re: [ontolog-forum] The Open world assumption shoe does not always fit -

[Pat Hayes <phayes@xxxxxxx>]

At 10:35 PM -0400 6/26/08, Bob Futrelle wrote:

> If I have a database of *all* employees
> in a company and a query for a
> person returns nothing, then that failure allows me to assert that
> that person is not an employee.

As long as you know that the database has that all-encompassing quality, yes. But then, since that knowledge is crucial to the conclusion being correct, it - the knowledge of the completeness - should be made explicit as a premis in the argument. And then the reasoning is classical.

Heres a way to say it. Logic is about what entails what. Now, suppose that:

John is not listed in database D of employees. Does it follow -is it entailed - that John is not an employee? Well, no, actually. But NAF would say that it does.

Now, in your scenario, we also know that the database is a list of ALL the employees. So add this as another assumption, since this is an important fact. NOW indeed it does follow that John is not an employee. But we don't need NAF to conclude this: it follows by classical logic from the two assumptions. Either way, in order to express the reasoning correctly, classical logic is exactly what we want. Using NAF on just the failure is an enthymeme: it has a missing premis, like saying "Plato is a man, so Plato is mortal", forgetting to add the 'obvious' premis that all men are mortal. As this example might suggest, this is a VERY old mistake in reasoning.

> It's a matter of deciding what
> your
> universe of discourse is, is it
> not?

Well, if your entire universe of discourse is just the employees of one company, then sure. But by and large, most ontologies have a rather wider view of the universe. And if you plan to publish stuff on a Web, then expecting everyone who reads it to agree with you that the entire universe comprises nothing but the employees listed in your database is asking rather a lot.

Bear in mind that the start of this thread was John Sowa's suggestion that CL be adopted as a basic notation for ontologies in general, a kind of super-OWL, and Adrian's response suggesting that there was something inherently wrong with classical negation for such a role.

Pat

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