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## [ontolog-forum] Time representation

 To: "[ontolog-forum]" "John F. Sowa" Sun, 20 Jan 2008 14:55:27 -0500 <4793A72F.8080304@xxxxxxxxxxx>
 ```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.    (01) John Sowa    (02) -------- Original Message --------    (03) There are many possible options for representing and reasoning about time. Any of them could be used with conceptual graphs.    (04) 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.    (05) 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.    (06) 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').    (07) An approach with explicit time represents time with a linear coordinate system. That raises more questions, whose answers require further distinctions:    (08) 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.    (09) 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.    (010) 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?    (011) 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.    (012) 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.    (013) 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.    (014) For further discussion, the following web page summarizes a taxonomy by Eric Sandewall with 2304 types of processes:    (015) http://www.jfsowa.com/ontology/process.htm    (016) 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:    (017) http://www.jfsowa.com/pubs/laws.htm Laws, Facts, and Contexts    (018) 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    (019) http://www.ihmc.us/users/phayes/IKL/SPEC/SPEC.html IKL Specification Document    (020) http://www.ihmc.us/users/phayes/IKL/GUIDE/GUIDE.html IKL Guide    (021) 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:    (022) [Proposition: p']    (023) where p' is the CG translation of p.    (024) Time does not apply directly to propositions, but to situations. In CGIF (Conceptual Graph Interchange Format), you could write    (025) [Situation *s] (PTim ?s, "20 January 2008") (Dscr ?s [Proposition p'])    (026) 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:    (027) (exists (s Situation) (and (PTim s "20 January 2008") (Dscr s (that p))))    (028) By using "type coercion", the above CGIF could be abbreviated to the following form:    (029) [Situation *s p'] (PTim ?s, "20 January 2008")    (030) 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.    (031) _________________________________________________________________ Message Archives: http://ontolog.cim3.net/forum/ontolog-forum/ Subscribe/Config: http://ontolog.cim3.net/mailman/listinfo/ontolog-forum/ Unsubscribe: mailto:ontolog-forum-leave@xxxxxxxxxxxxxxxx Shared Files: http://ontolog.cim3.net/file/ Community Wiki: http://ontolog.cim3.net/wiki/ To Post: mailto:ontolog-forum@xxxxxxxxxxxxxxxx    (032) ```
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