# Re: RDF 1.1: "Some properties may change over time." (ISSUE-178)

```On Dec 10, 2013, at 4:24 PM, Markus Lanthaler <markus.lanthaler@gmx.net> wrote:

> On Tuesday, December 10, 2013 8:31 PM, Thomas Baker wrote:
>> On Tue, Dec 10, 2013 at 06:08:29PM +0100, Markus Lanthaler wrote:
>>> I think something like
>>>
>>>    A relationship that holds between two resources at one time
>>>    (i.e., an RDF statement) may not hold at another time.
>>
>> Hmm, but could this wording be read as implying that a statement
>> actually _is_ a relationship? It's more like a statement asserts a
>> relationship.
>
> Hmm... yeah, maybe.
>
>
>>> or even just
>>>
>>>    An RDF statement that holds at one time may not hold at another
>>>    time.
>>
>> However, an RDF statement would continue to "hold" its meaning, even if
>> its
>> relationship to reality were to change (because reality changes).  The
>> challenge is to avoid the impression that properties or statements are
>> themselves mutable, as opposed to the relationship between two
>> resources.  This
>> can perhaps be avoided if it's the "relationship between two resources"
>>
>>      A relationship that holds between two resources at one time, as
>>      asserted by an RDF statement, may not hold at another time.
>>
>> or
>>
>>      A relationship that holds between two resources, as asserted by
>>      an RDF statement at one time, may not hold at another time.
>
> Works for me. We could perhaps make it even simpler by just saying
>
>   A relationship that holds between two resources at one time
>   may not hold at another time.
>
>

This last one is the least bad of the lot. But none of them are correct. There is a basic issue here. Just like sets, relations cannot really change with time. At least, not when they are described using a normal logic (they can in a tense logic). What can happen is that something that we might casually or carelessly describe as a binary relation is in fact a three-way relation with time as its third argument. Now of course [ R(a, b) at T ] or R(a, b, T) pretty much mean the same thing and in English we don't even have a way to distinguish them; but being all logical and strict about it, the three-argument way of talking is more accurate precisely because it makes it clear that the *actual relation* does not change, which makes sense because relations (speaking now formally and mathematically proper), like sets, just aren't the kind of thing that can possibly change. (If this reminds y'all of the problems we had with talking about RDF graphs being updated or modified, yes it is exactly the same issue.) We could have made RDF into a tensed logic, in which all assertions are made at a time, and things like a triple being true AT a time would make literal sense; but we didn't. So right now, and probably for the forseeable future, the idea of a relation changing - holding at one time but not at another time - does not make sense according to the RDF conceptual model, so temporal variation like this has to be modeled in the same way we would model a three-place relation in RDF.

We might say something like this:

Some relations have an extra time parameter or are time-dependent. Such a relationship that holds between two resources at one time might not hold at another time. To describe this in RDF we have to treat the time as an extra argument or parameter to the binary relation.

Pat

> Thoughts?
>
>
> --
> Markus Lanthaler
> @markuslanthaler

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Received on Wednesday, 11 December 2013 05:29:30 UTC