Re: Blank nodes must DIE! [ was Re: Blank nodes semantics - existential variables?]

On 7/8/2020 10:51 AM, Nicolas Chauvat wrote:
> On Wed, Jul 08, 2020 at 08:28:05AM -0400, Thomas Passin wrote:
>> the same points if they are not (and yes, I know that it could be considered
>> a nice philosophical question whether they are "really" the same, but let's
>> not enter in that here).
> 
> Isn't it the whole point of the current (sub)discussion ?
> Would you say that in the real world there are several different
> literals that can have the value "hello" or would you say that two
> literals that have the same value can be considered to be the same ?

Here's what I am thinking of, and I'll talk about points, since that was 
the example I responded to (well, I think it was actually about the 
center of the circle, but never mind).  Think of a data point with a 
specific (x,y) value for the central location, one point of a set of 
data points, and let's say that that value is (1, 2).  The point might 
have had a value of (1.001, 1.999), but perhaps because of round-off 
error or some quantization procedure, the value recorded data tuple is 
(1.0, 2.0). or even (1, 2), by which I mean (int(1), int(2)).

Now consider another data set, one of whose data points also has the 
data tuple (1, 2).  I am going to claim that those two data points, 
members of two different data sets, are not necessarily the same points 
even if they end up being represented by the same data tuple - (1, 2) in 
this case.  In physics, maybe the tuple values represent energy levels 
that change when a magnetic field is applied, but are degenerate when 
there is no field.  In this case they happen to be the same as each 
other because no field was applied to the one case, and in the other 
case the field would not have changed the value.

You may say that this is a contrived case because the points in question 
originally were mathematical points, and they are characterized by pair 
of 2-D coordinates.  My point, though, is that in the context of an RDF 
graph (not to mention "real life"), any particular thing (or call it a 
resource, subject, or what have you) may have any number of additional 
complexities associate with it even if you don't happen to have included 
them in your graph.  That's the good old open world assumption.

So unless you truly know that two point nodes with the same coordinate 
tuple of (1, 2) are the representing the very same thing, they may very 
well be different - in the context of RDF and the open world assumption. 
  It's not about a philosophical difference.

Or put another way, just because the same literal value is assigned to 
two nodes does not make them the same.  Is that literal value itself the 
"same" for both?  In some software it might be - if it's been memoized, 
for example - but that's normally an implementation detail, not 
something fundamental.

TomP


> https://en.wikipedia.org/wiki/String_interning shows that some
> programming languages store only one copy of immutable strings (or
> immutable composite values/immutable structs in some cases) even when
> these strings that have the same value are created at different places
> and times in the program.
> 
> Why would you call that question "philosophical" ? Do you mean it is
> irrelevant ? If you think it is irrelevant, could you explain why ?
>

Received on Wednesday, 8 July 2020 15:54:34 UTC