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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