Examples of Syntax and semantics

Henry,
It seems rather late in the day to be defining the diference between syntax
and semantics, with regard to which imagine the following.
A painting is fully described in mathematical terms. You can go to any level
of complexity, devise any scheme you like to describe the texture of the
paint, the juxtaposition of colours, what you will to *completely* describe
it, but the language and its syntax is so at odds to that of painting you
will not be able to convey the meaning of the painting to another human in
this way.
So it is, less dramatically, between one ontology and another although
ostensibly about the same subject. Without an underlying framework what are
such ontologies predicated on? Trial and error in the popularity stakes?
You said:-

> <http://xmlns.com/foaf/0.1/Person> (Let's abbreviate that to
> foaf:Person from here on) always refers to the class of persons.
> foaf:knows always to the same relation between persons.


And I understand that the point is that the a foaf person in defined is
available to all at the URL.
But the problem comes when I want to redefine that which would manifest in
either extending http://xmlns.com/foaf/0.1/Person or  by my own definition
http://xmlns.com/asaltieldeffoaf/0.1/Person<http://xmlns.com/foaf/0.1/Person>
or, worse, where there is an implicit change. In fact this has happened in
foaf:knows since the way this is used entails a very attenuated sense of
"knows" due to foaf based programs (e.g. LinkedIn). One certainly cannot
rely on foaf:knows in the way that one ordinarily uses the term "I know June
and I know Justin."
(And notice the shades of meaning that can be attached to "I know June" or,
again, "I know June and Justin.") So I may wish intenstionally to adjust
foaf to accommodate some of these shades of meaning.
I then may mistakenly think that I can normalize
http://xmlns.com/asaltieldeffoaf/0.1/Person to
http://xmlns.com/foaf/0.1/Person by mapping some essential property of my
definition of knows to foaf:knows and leaving the other properties as (? I
don't know the technical details but I will imagine this) anonymous
properties in foaf effectively extending foaf to my own purpose but in an
unsatisfactory way.
What would have happened at this point is that my original meaning would be
lost, this is because foaf:knows has its own meaning, due to usage as much
as anything, and my sense of asaltieldeffoaf:knows such as knows as good
friend, knows as in the brief acquaintance of a collegue and so on, would
not fit into foaf:knows.
You said:-

> The trick is that the names you use (URIs) from one language to the
> other are always the same. The semantics is captured in the
> relationship between the names and the things (which is what
> semantics is about).


So how would this work in the example I have given?
Best,
Adam