thanks a lot, most important (01)
Following on the fact that the ability of representing 'temporal'
properties may be desirable
in a semantic data model, does your conclusion recommend that IKL is used as
representation of K (or data) on the web rather than RDF/OWL, or in
combination with rdf/owl
or what exactly shall we do? (02)
On Jan 21, 2008 2:55 AM, John F. Sowa <sowa@xxxxxxxxxxx> wrote:
> 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:
> 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:
> 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
> IKL Specification Document
> 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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Paola Di Maio
School of IT
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