Denny, I didn't elaborate because Emw mostly did so for me. But since you asked, I will do so as well. Let's start with your examples.
The allowed value for the "country" property should not be "goat cheese". The allowed values should be those items that contain the statement "instance of: country". And you'll notice that this allows ambiguity, because even though Kosovo is not a country according to Serbia, it is a country according to some other countries. Since Wikidata is not about the truth, but about statements and their sources, we can record that a certain country stated that Kosovo is a country, just as we can also record that Serbia stated that Kosovo is a region within Serbia. And yes, Wikidata would also need built-in "instance of" and "subclass of" properties.
Does every country have a capital city? Emw mentioned Nauru, which doesn't. Well, Wikidata already covers that - the "no value" special property value.
Why isn't it sensible to restrict the domains of properties as well as their ranges? The domain of "capital city" should be restricted to those items that are instances of countries (or countries union administrative divisions, not sure). That is, after all, part of the semantics of the "capital city" concept--why shouldn't we be able to capture those semantics?
Can authorship apply to a song? I haven't yet heard the argument that it can't. But if someone claims it can't, and others claim it can, obviously they have different definitions of the concept of authorship. And that is fine, because those different definitions can be captured by different properties. What's wrong with that?
What's wrong with ontology engineering? You said you think that it can get stuck in a fundamentally unresolvable situation, how and why?
What I'm mainly disappointed about is that your opinion in the article, and thus the design of Wikidata, is presented matter-of-factly, and not as an invitation to a discussion. Don't you think anyone beyond the Wikidata team should have some input regarding these matters?
John H., I am sorry to do so. I really should be more conscious about the environment and recycle more.
Or am I disappointing you in some other way? It would be helpful if you actually elaborate a little bit.
"The world is complex. Wikidata aims to collect structured knowledge about this complex world."
The world is complex, but it has structure. Classes or types are a useful way to express that structure. All knowledge representation technologies that I'm aware of -- RDF, RDFS, OWL, UML, etc. -- support statements about a subject's class. As an important development in knowledge representation, shouldn't Wikidata also support class relations?
How many Roman senators were donkeys? How many countries don't have capitals? I think the proportion of such class exceptions to conforming subjects is very low. (There was a horse, Incitatus, who was said to be treated as a Roman consul, and Nauru is the only country without an official capital; in both cases, the proportion of such exceptions in those classes is well under 1%.) For subjects with disputed classes, why not allow users to specify different classes for a subject, and in those cases relax certain property restrictions? Even then, while there is dispute as to whether, for example, Kosovo is an independent country, everyone agrees that its area cannot be 60 minutes.
I think the overarching promise of Wikidata is as a centralized repository of structured data for Wikipedia. Without support for classes, Wikidata might be slightly more flexible, but it would be much less structured.
I strongly agree. SAP class business systems would need considerably less customization without a myrid of restrictive data features, and conversly, bennefit considerally from bots checking for inconsistencies.
On ACID, Amazon's Dynamo paper, has proven availability trumps consistency, its good to see the same thinking propogated all the way to freedom of the users application.
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Denny, I didn't elaborate because Emw mostly did so for me. But since you asked, I will do so as well. Let's start with your examples. The allowed value for the "country" property should not be "goat cheese". The allowed values should be those items that contain the statement "instance of: country". And you'll notice that this allows ambiguity, because even though Kosovo is not a country according to Serbia, it is a country according to some other countries. Since Wikidata is not about the truth, but about statements and their sources, we can record that a certain country stated that Kosovo is a country, just as we can also record that Serbia stated that Kosovo is a region within Serbia. And yes, Wikidata would also need built-in "instance of" and "subclass of" properties. Does every country have a capital city? Emw mentioned Nauru, which doesn't. Well, Wikidata already covers that - the "no value" special property value. Why isn't it sensible to restrict the domains of properties as well as their ranges? The domain of "capital city" should be restricted to those items that are instances of countries (or countries union administrative divisions, not sure). That is, after all, part of the semantics of the "capital city" concept--why shouldn't we be able to capture those semantics? Can authorship apply to a song? I haven't yet heard the argument that it can't. But if someone claims it can't, and others claim it can, obviously they have different definitions of the concept of authorship. And that is fine, because those different definitions can be captured by different properties. What's wrong with that? What's wrong with ontology engineering? You said you think that it can get stuck in a fundamentally unresolvable situation, how and why? What I'm mainly disappointed about is that your opinion in the article, and thus the design of Wikidata, is presented matter-of-factly, and not as an invitation to a discussion. Don't you think anyone beyond the Wikidata team should have some input regarding these matters?
John H., I am sorry to do so. I really should be more conscious about the environment and recycle more. Or am I disappointing you in some other way? It would be helpful if you actually elaborate a little bit.
Denny, you disappoint me.
"The world is complex. Wikidata aims to collect structured knowledge about this complex world." The world is complex, but it has structure. Classes or types are a useful way to express that structure. All knowledge representation technologies that I'm aware of -- RDF, RDFS, OWL, UML, etc. -- support statements about a subject's class. As an important development in knowledge representation, shouldn't Wikidata also support class relations? How many Roman senators were donkeys? How many countries don't have capitals? I think the proportion of such class exceptions to conforming subjects is very low. (There was a horse, Incitatus, who was said to be treated as a Roman consul, and Nauru is the only country without an official capital; in both cases, the proportion of such exceptions in those classes is well under 1%.) For subjects with disputed classes, why not allow users to specify different classes for a subject, and in those cases relax certain property restrictions? Even then, while there is dispute as to whether, for example, Kosovo is an independent country, everyone agrees that its area cannot be 60 minutes. I think the overarching promise of Wikidata is as a centralized repository of structured data for Wikipedia. Without support for classes, Wikidata might be slightly more flexible, but it would be much less structured.
Nonthless there is value in ontological checking of one's belief system as Clive says above. This is only part of error control though.
I strongly agree. SAP class business systems would need considerably less customization without a myrid of restrictive data features, and conversly, bennefit considerally from bots checking for inconsistencies. On ACID, Amazon's Dynamo paper, has proven availability trumps consistency, its good to see the same thinking propogated all the way to freedom of the users application.
A fascinating post - thanks for sharing, Denny!