Dear John, (01)
I know you know but did not mention: (02)
> This list of options illustrates why it is so difficult to
> handle time in RDF and OWL: they have no way of representing
> contexts, extra arguments for time, or modal and temporal
> operators. For a specific problem, some ad hoc solution may
> be possible, but there is no systematic representation. (03)
These difficulties do not apply to 4-dimensionalism, because
periods in time are 4D spatio-temporal extents, and you can
simply say that a process, or a state was part of that period.
This can be handled by either OWL or CL. (04)
Regards (05)
Matthew West
Reference Data Architecture and Standards Manager
Shell International Petroleum Company Limited
Registered in England and Wales
Registered number: 621148
Registered office: Shell Centre, London SE1 7NA, United Kingdom (06)
Tel: +44 20 7934 4490 Mobile: +44 7796 336538
Email: matthew.west@xxxxxxxxx
http://www.shell.com
http://www.matthew-west.org.uk/ (07)
> -----Original Message-----
> From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx
> [mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx]On Behalf Of
> John F. Sowa
> Sent: 20 January 2008 19:55
> To: [ontolog-forum]
> Subject: [ontolog-forum] Time representation
>
>
> On the CG forum, there was a question about representing time in
> conceptual graphs and OWL. Since that topic may be of interest
> to people on this forum, I am forwarding the following copy of
> my response.
>
> John Sowa
>
> -------- Original Message --------
>
> There are many possible options for representing and reasoning
> about time. Any of them could be used with conceptual graphs.
>
> OWL is an extremely limited subset of logic, which was not
> designed to accommodate time in a systematic way. In fact, the
> purpose of OWL was to represent a limited subset of logic that
> is used for stating timeless relationships among the categories
> of an ontology. Trying to add time to such a logic requires
> either a complete overhaul of the entire system or an ad hoc
> solution that is useful for only a single type of problem.
>
> In representing time, the first option is to choose between an
> explicit time or an implicit "temporal logic", which does not
> actually refer to time.
>
> Temporal logics avoid references to time by using operators
> such as 'sometimes', 'always', 'before', and 'after'. That
> representation, which was developed by Arthur Prior (a good
> name for a philosopher who was writing about time), treats
> time as a kind of modality (with 'always' corresponding
> to 'necessity' and 'sometimes' to 'possibility').
>
> An approach with explicit time represents time with a linear
> coordinate system. That raises more questions, whose answers
> require further distinctions:
>
> a) 4 dimensions or 3+1 dimensions? A 4-D approach treats objects
> and processes as connected regions of a four-dimensional
> space-time continuum. A 3+1 D approach treats space and time
> as independent, but related, coordinate systems.
>
> b) Time points or intervals? Using real numbers to represent time
> coordinates implies that time is divisible into infinitely small
> points. But finite intervals with domain-dependent granularity
> are more realistic.
>
> c) Contexts or extra arguments on relations? If time is represented
> by some coordinate system, how are those coordinates associated
> with the other representations? By attaching the time (and/or
> space) coordinate to a context box or other delimiter
> that encloses
> the description of a situation at that time? Or by adding another
> argument to every relation to indicate the time when it is true?
>
> As an example of point (c), the relation HasPart(x,y) would say
> that x has y as part. If parts can be added or lost over time,
> there must be some way to state when that relation is true.
>
> The first option specifies the point in time t of a situation
> when all the relationships happened to be true, including the
> fact that some entity x had some entity y as part. The second
> option adds an extra argument to every relation; for example,
> HasPart(x,y,t). The first option adds more complexity to
> the logic, but simplifies the description of each situation.
> The second option adds more arguments to every relation, but
> it does not change the underlying logic.
>
> This list of options illustrates why it is so difficult to
> handle time in RDF and OWL: they have no way of representing
> contexts, extra arguments for time, or modal and temporal
> operators. For a specific problem, some ad hoc solution may
> be possible, but there is no systematic representation.
>
> For further discussion, the following web page summarizes a
> taxonomy by Eric Sandewall with 2304 types of processes:
>
> http://www.jfsowa.com/ontology/process.htm
>
> The following paper describes how a representation with time
> (or other kind of index) attached to a context box could be
> mapped to a flat representation with the index added as an
> extra argument to each relation:
>
> http://www.jfsowa.com/pubs/laws.htm
> Laws, Facts, and Contexts
>
> Common Logic does not make provision for delimiting a
> description and referring to it in some other statement.
> But the IKL extension to Common Logic does. See
>
> http://www.ihmc.us/users/phayes/IKL/SPEC/SPEC.html
> IKL Specification Document
>
> http://www.ihmc.us/users/phayes/IKL/GUIDE/GUIDE.html
> IKL Guide
>
> The IKL language adds an expression type '(that p)' to
> Common Logic, where p is arbitrary sentence that states
> some proposition. That expression maps to a CG context
> box of the following form:
>
> [Proposition: p']
>
> where p' is the CG translation of p.
>
> Time does not apply directly to propositions, but to situations.
> In CGIF (Conceptual Graph Interchange Format), you could write
>
> [Situation *s] (PTim ?s, "20 January 2008") (Dscr ?s [Proposition p'])
>
> This says that there exists a situation s whose point in time is
> 20 January 2008 and whose description is the proposition p'. It
> could be translated to the following statement in IKL:
>
> (exists (s Situation)
> (and (PTim s "20 January 2008") (Dscr s (that p))))
>
> By using "type coercion", the above CGIF could be abbreviated
> to the following form:
>
> [Situation *s p'] (PTim ?s, "20 January 2008")
>
> Whenever a conceptual graph, such as p', is nested inside a box
> of any type other than Proposition, the default assumption is
> that the CG describes some entity of that type. Therefore, this
> abbreviated notation can be expanded to the above CGIF statement.
>
>
>
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> (08)
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