This one point requires a tiny remark: there is no problem in OWL or RDF to use the same URI as a property, an individual, and a class in different contexts. The only thing that OWL (DL) forbids is to use one property for literal values (like string) and for object values (like other items), but this would not occur in our case anyway since we have clearly defined types. I completely agree with all the rest :-)

Key differences between Properties and Items: * Properties have a data type, items don't. * Items have sitelinks, Properties don't. * Items have Statements, Properties will support Claims (without sources). The software needs these constraints/guarantees to be able to take shortcuts, provide specialized UI and API functionality, etc. Yes, it would be possible to use items as properties instead of having a separate entity type. But they are structurally and functionally different, so it makes sense to have a strict separate. This makes a lot of things easier, e.g.: * setting different permissions for properties * mapping to rdf vocabularies More fundamentally, they are semantically different: an item describes a concept in "the real world", while a property is a structural component used for such a description. Yes, properies are simmilar to data items, and in some cases, there may be an item representing the same concept that is represented by a property entity. I don't see why that is a problem, while I can see a lot of confusion arising from mixing them. -- daniel Am 28.05.2014 09:25, schrieb David Cuenca: -- Daniel Kinzler Senior Software Developer Wikimedia Deutschland Gesellschaft zur Förderung Freien Wissens e.V. _______________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikidata-l

Héhé, the Wikidata game suggest it may be a little bit too complicated and better abstracted away by a three button game for mass contribution :) 2014-05-29 21:04 GMT+02:00 Andrew Gray <andrew.gray(a)dunelm.org.uk>uk>:

On 29/05/14 21:04, Andrew Gray wrote: Good point. The thread has really gone off in a rather philosophical direction :-) As Jane said, examples (of places where a property should be used *and* of places where it should not be used) are definitely much more useful to help our editors on the ground. I usually use items I know as role models or have a look for suitable showcase items.

Do we have an easy way of highlighting a gallery of good examples or even a plain wikipage of topical guidance? Would be very useful if we could say 'here's a politician, here's a French city, etc'

Hi David, Interesting remark. Let's explore this idea a bit. I will give you two main reasons why we have properties separate, one practical and one conceptual. First the practical point. Certainly, everything that is used as a property needs to have a datatype, since otherwise the wiki would not know what kind of input UI to show. So you cannot use just any item as a property straight away -- it needs to have a datatype first. So, yes, you could abolish the namespace Property but you still would have a clear, crisp distinction between property items (those with datatype) and normal items (those without a datatype). Because of this, most of the other functions would work the same as before (for example, property autocompletion would still only show properties, not arbitrary items). A complication with this approach is that property datatypes cannot change in Wikibase. This design was picked since there is no way to convert existing data from one datatype to another in general. So changing the datatype would create problems by making a lot of data "invalid", and require special handling and special UI to handle this situation. With properties living in a separate namespace, this is not a real restriction: you can just create a new property and give it the same label (after naming the old one differently, e.g., putting "DEPRECATED" in its name). Then you can migrate the data in some custom fashion. But if properties would be items, we would have a problem here: the item is already linked to many Wikipedias and other projects, and it might be used in LUA scripts, queries, or even external applications like Denny's Javascript translation library. You cannot change item ids easily. Also, many items would not have a datatype, so the first one who (accidentally?) is entered will be fixed. So we would definitely need to rethink the whole idea of unchangeable datatypes. My other important reason is conceptual. Properties are not considered part of the (encyclopaedic) data but rather part of the schema that the community has picked to organise that data. As in your example, "emissivity" (Q899670) is a notion in physics as described in a Wikipedia article. There are many things to say about this notion (for example, it has a history: somebody must have defined this first -- although Wikipedia does not say it in this case). As in all cases, some statements might be disputed while others are widely acknowledged to be "true". For the property "emissivity" (P1295), the situation is quite different. It was introduced as an element used to enter data, similar to a row in a database table or an infobox template in some Wikipedia. It does probably closely relate to the actual physical notion Q899670, but it still is a different thing. For example, it was first introduced by User:Jakec, who is probably not the person who introduced the physical concept ;-) Anything that we will say about P1295 in the future refers to the property -- a concept of our own making, that is not described in any external source (there are no publications discussing P1295). This is also the reason why properties are supposed to support *claims* not *statements*. That is, they will have property-value pairs and qualifiers, but no references or ranks. Indeed, anything we say about properties has the status of a definition. If we say it, it's true. There is no other authority on Wikidata properties. You could of course still have items and properties "share" a page and somehow define which statements/claims refer to which concept, but this does not seem to make things easier for users. These are, for me, the two main reasons why it makes sense to keep properties apart from items on a technical level. Besides this, it is also convenient to separate the 1000-something properties from the 15-million something items for reasons of maintenance. Best regards, Markus On 28/05/14 09:25, David Cuenca wrote: _______________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikidata-l

David, Regarding the question of how to classify properties and how to relate them to items: * "same as" (in the sense of owl:sameAs) is not the right concept here. In fact, it has often been discouraged to use this on the Web, since it has very strong implications: it means that in all uses of the one identifier, one could just as well use the other identifier, and that it is indistinguishable if something has been said about the one or the other. That seems too strong here, at least for most cases. * In the world of OWL DL, sameAs specifically refers to individuals, not to classes or properties. Saying "P sameAs Q" does not imply that P and Q have the same extension as properties. For the latter, OWL has the relationship owl:equivalentProperties. This distinction of instance level and schema level is similar to the distinction we have between "instance of" and "subclass of". * Therefore, I would suggest to use a property called "subproperty of" as one way of relating properties (analogously to "subclass of"). It has to be checked if this actually occurs in Wikidata (do we have any properties that would be in this relation, or do we make it a modelling principle to have only the most specific properties in Wikidata?). * The relationship from properties to items could be modelled with the existing property "subject of" (P805). * It might be useful to also have a taxonomic classification of properties. For example, we already group properties into properties for "people", "organisations", etc. Such information could also be added with a specific property (this would be a bit more like a "category" system on property pages).

On the other hand, some of this might coincide with constraint information that could be expressed as claims. For instance, person properties might be those with "Type" (i.e., "rdfs:domain") constraint human. By the way, our constraint system could use some systematisation -- there are many overlaps in what you can do with one constraint or another.

Markus, Ok, now I understand that "same as" wouldn't be a good name for the confusion it would cause. However the property "subject of" as it is now wouldn't be a good candidate either. Its meaning is that a certain statement is represented by another item (that is why it is only allowed to be used as qualifier).

Perhaps a better name would be "corresponds with item" and the inverse "corresponds with property". Just by having these connections, a lot of information can be inferred from the connected item. Consider the following example with "occupation (P106)", and "occupation (Q13516667)": - I cannot find any clear "subproperty of" for p106, but there is a clear "subclass of:human behaviour" for the item - "human behaviour" is "part of" human

- "human" can have a statement "intrinsic property" (property proposal still under discussion) with values "birthday (Q47223)" and an "(eventual) date of death". It can be expanded in the future to include newly created properties like "height", "weight", "eye color", etc

- birthday (Q47223) <corresponds with property> date of birth (P569)

Out of this I reach the following conclusions: - the taxonomy of properties is going to be weak, since there is not always a clear subpropertyOf unless created artificially (more work)

- the standard taxonomy of items (subclass of/part of) is sufficient to automatically reach meaningful constraints and inference (less work)

- by adding manually the constraints to the property itself we are duplicating information which will require volunteer effort to maintain (more work)

My recommendation is to rely mainly on the main taxonomy instead of creating a parallel property taxonomy, and then think of ways to extract information from the main taxonomy to convert it automatically into constraints. All the maintenance takes effort, so the more it can be automated, the more efficient volunteers will be. And if we can simplify the maintenance of properties, we will be able to simplify the creation of properties too, specially when we face the next surge which will come with the datatype "number with units".

On Wed, May 28, 2014 at 2:48 PM, Markus Krötzsch &lt;markus(a)semantic-mediawiki.org <mailto:markus@semantic-mediawiki.org>> wrote: David, Regarding the question of how to classify properties and how to relate them to items: * "same as" (in the sense of owl:sameAs) is not the right concept here. In fact, it has often been discouraged to use this on the Web, since it has very strong implications: it means that in all uses of the one identifier, one could just as well use the other identifier, and that it is indistinguishable if something has been said about the one or the other. That seems too strong here, at least for most cases. * In the world of OWL DL, sameAs specifically refers to individuals, not to classes or properties. Saying "P sameAs Q" does not imply that P and Q have the same extension as properties. For the latter, OWL has the relationship owl:equivalentProperties. This distinction of instance level and schema level is similar to the distinction we have between "instance of" and "subclass of". * Therefore, I would suggest to use a property called "subproperty of" as one way of relating properties (analogously to "subclass of"). It has to be checked if this actually occurs in Wikidata (do we have any properties that would be in this relation, or do we make it a modelling principle to have only the most specific properties in Wikidata?). * The relationship from properties to items could be modelled with the existing property "subject of" (P805). * It might be useful to also have a taxonomic classification of properties. For example, we already group properties into properties for "people", "organisations", etc. Such information could also be added with a specific property (this would be a bit more like a "category" system on property pages). On the other hand, some of this might coincide with constraint information that could be expressed as claims. For instance, person properties might be those with "Type" (i.e., "rdfs:domain") constraint human. By the way, our constraint system could use some systematisation -- there are many overlaps in what you can do with one constraint or another. Cheers, Markus On 28/05/14 12:14, David Cuenca wrote: Markus, The explanation about the implications of renaming/deleting makes most sense and just that justifies already the separation in two. It is equally true that when we create a property, we might have "cleaned" the original concept so much that it might differ (even slightly) with the understood concept that the item represents. However, even after that process, the "new" concept is still an item... The process of imbuing a concept with permanent characteristics (adding a datatype) and the practical approach, also seems to recommend keeping items and properties separate. Thanks for showing me that reasoning :) I am still wondering about how are we going to classify properties. Maybe it will require a broader discussion, but if they are the same (or mostly the same) as items, then we can just link them as "same as", and build the classing structure just for the items. OTOH, if they are different, then we will need to mirror that classification for properties, which seems quite redundant. Plus adding a new datatype, "property". All in all, my conclusion about this is that properties are just concepts with special qualities that justify the separation in the software (even if in real life there is no separation). many thanks for your detailed answer, and sorry if I'm bringing up already discussed topics. It is just that when you stare long into wikidata, wikidata stares back into you ;) Cheers, Micru On Wed, May 28, 2014 at 11:39 AM, Markus Krötzsch &lt;markus(a)semantic-mediawiki.org <mailto:markus@semantic-mediawiki.org> <mailto:markus@semantic-__mediawiki.org <mailto:markus@semantic-mediawiki.org>>> wrote: Hi David, Interesting remark. Let's explore this idea a bit. I will give you two main reasons why we have properties separate, one practical and one conceptual. First the practical point. Certainly, everything that is used as a property needs to have a datatype, since otherwise the wiki would not know what kind of input UI to show. So you cannot use just any item as a property straight away -- it needs to have a datatype first. So, yes, you could abolish the namespace Property but you still would have a clear, crisp distinction between property items (those with datatype) and normal items (those without a datatype). Because of this, most of the other functions would work the same as before (for example, property autocompletion would still only show properties, not arbitrary items). A complication with this approach is that property datatypes cannot change in Wikibase. This design was picked since there is no way to convert existing data from one datatype to another in general. So changing the datatype would create problems by making a lot of data "invalid", and require special handling and special UI to handle this situation. With properties living in a separate namespace, this is not a real restriction: you can just create a new property and give it the same label (after naming the old one differently, e.g., putting "DEPRECATED" in its name). Then you can migrate the data in some custom fashion. But if properties would be items, we would have a problem here: the item is already linked to many Wikipedias and other projects, and it might be used in LUA scripts, queries, or even external applications like Denny's Javascript translation library. You cannot change item ids easily. Also, many items would not have a datatype, so the first one who (accidentally?) is entered will be fixed. So we would definitely need to rethink the whole idea of unchangeable datatypes. My other important reason is conceptual. Properties are not considered part of the (encyclopaedic) data but rather part of the schema that the community has picked to organise that data. As in your example, "emissivity" (Q899670) is a notion in physics as described in a Wikipedia article. There are many things to say about this notion (for example, it has a history: somebody must have defined this first -- although Wikipedia does not say it in this case). As in all cases, some statements might be disputed while others are widely acknowledged to be "true". For the property "emissivity" (P1295), the situation is quite different. It was introduced as an element used to enter data, similar to a row in a database table or an infobox template in some Wikipedia. It does probably closely relate to the actual physical notion Q899670, but it still is a different thing. For example, it was first introduced by User:Jakec, who is probably not the person who introduced the physical concept ;-) Anything that we will say about P1295 in the future refers to the property -- a concept of our own making, that is not described in any external source (there are no publications discussing P1295). This is also the reason why properties are supposed to support *claims* not *statements*. That is, they will have property-value pairs and qualifiers, but no references or ranks. Indeed, anything we say about properties has the status of a definition. If we say it, it's true. There is no other authority on Wikidata properties. You could of course still have items and properties "share" a page and somehow define which statements/claims refer to which concept, but this does not seem to make things easier for users. These are, for me, the two main reasons why it makes sense to keep properties apart from items on a technical level. Besides this, it is also convenient to separate the 1000-something properties from the 15-million something items for reasons of maintenance. Best regards, Markus On 28/05/14 09:25, David Cuenca wrote: Since the very beginning I have kept myself busy with properties, thinking about which ones fit, which ones are missing to better describe reality, how integrate into the ones that we have. The thing is that the more I work with them, the less difference I see with normal items.... and if soon there will be statements allowed in property pages, the difference will blur even more. I can understand that from the software development point of view it might make sense to have a clear difference. Or for the community to get a deeper understanding of the underlying concepts represented by words. But semantically I see no difference between: cement (Q45190) <emissivity (P1295)> 0.54 and cement (Q45190) <emissivity (Q899670)> 0.54 Am I missing something here? Are properties really needed or are we adding unnecessary artificial constraints? Cheers, Micru ___________________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org <mailto:Wikidata-l@lists.wikimedia.org> <mailto:Wikidata-l@lists.__wikimedia.org <mailto:Wikidata-l@lists.wikimedia.org>> https://lists.wikimedia.org/____mailman/listinfo/wikidata-l <https://lists.wikimedia.org/__mailman/listinfo/wikidata-l> <https://lists.wikimedia.org/__mailman/listinfo/wikidata-l <https://lists.wikimedia.org/mailman/listinfo/wikidata-l>> ___________________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org <mailto:Wikidata-l@lists.wikimedia.org> <mailto:Wikidata-l@lists.__wikimedia.org <mailto:Wikidata-l@lists.wikimedia.org>> https://lists.wikimedia.org/____mailman/listinfo/wikidata-l <https://lists.wikimedia.org/__mailman/listinfo/wikidata-l> <https://lists.wikimedia.org/__mailman/listinfo/wikidata-l <https://lists.wikimedia.org/mailman/listinfo/wikidata-l>> -- Etiamsi omnes, ego non _________________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org <mailto:Wikidata-l@lists.wikimedia.org> https://lists.wikimedia.org/__mailman/listinfo/wikidata-l <https://lists.wikimedia.org/mailman/listinfo/wikidata-l> _________________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org <mailto:Wikidata-l@lists.wikimedia.org> https://lists.wikimedia.org/__mailman/listinfo/wikidata-l <https://lists.wikimedia.org/mailman/listinfo/wikidata-l> -- Etiamsi omnes, ego non _______________________________________________ Wikidata-l mailing list Wikidata-l(a)lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikidata-l