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

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