Temporal Representation and Reasoning in OWL 2

Tracking #: 1118-2330

Sotiris Batsakis
Euripides Petrakis
Ilias Tachmazidis
Grigoris Antoniou

Responsible editor: 
Aldo Gangemi

Submission type: 
Full Paper
The representation of temporal information has been in the center of intensive research activities over the years in the areas of knowledge representation, databases and more recently, the Semantic Web. The proposed approach extends the existing framework of representing temporal information in ontologies by allowing for representation of concepts evolving in time (referred to as “dynamic” information) and of their properties in terms of qualitative descriptions in addition to quantitative ones (i.e., dates, time instants and intervals). For this purpose, we advocate the use of natural language expressions, such as “before” or “after”, for temporal entities whose exact durations or starting and ending points in time are unknown. Reasoning over all types of temporal information (such as the above) is also an important research problem. The current work addresses all these issues as follows: The representation of dynamic concepts is achieved using the “4D-fluents” or, alternatively, the “N-ary relations” mechanism. Both mechanisms are thoroughly explored and are expanded for representing qualitative and quantitative temporal information in OWL. In turn, temporal information is expressed using either intervals or time instants. Qualitative temporal information representation in particular, is realized using sets of SWRL rules and OWL axioms leading to a sound, complete and tractable reasoning procedure based on path consistency applied on the existing relation sets. Building upon existing Semantic Web standards, tools and recommendations (like OWL 2, SWRL), as well as integrating temporal reasoning support into the proposed representation, are important design features of our approach.
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Minor Revision

Solicited Reviews:
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Review #1
By Anisa Rula submitted on 21/Aug/2015
Minor Revision
Review Comment:

Though the revised version of the paper is improved, and many of the
detailed comments have been addressed, I still think more work is
needed since the broader comments still hold. I'll try to give a
more detailed idea of my remaining concerns than I did in my
previous review.

A general comment is about the experimentation. Although I see an
improvement with respect to the previous version of the paper I
still don’t understand what about large datasets containing more
that 500 time points or time intervals? I understand that the
authors will consider it as a future work (Section conclusion) but I
would expect a more detailed discussion about this point since it is
quite relevant nowdays.

Section 1
* State clearly and explicitly what are your contributions by
enumerating them before the outline of the paper. In addition, you
should also state that this work builds on a previous work and
describe with one sentence what you presented in the previous work
and provide a reference to it. Then, you should state explicitly how
this work extends the previous work.
* “In our approach, SWRL and OWL 2 constructs (e.g., disjoint
properties) are combined, offering a sound and complete reasoning
procedure ensuring path consistency [31], an issue which is not
examined in the original work by Welty and Fikes or by other known
approaches for temporal information representation (e.g., [2], [15],
[19], [8], [13]), while CNTRO ontology [39] is not combined with a
sound and complete reasoning mechanism over interval Allen relations
(reasoning over points and timestamps is supported in CNTRO) as this
work does.” -> Rephrase this sentence since it is too confusing by
splitting it in shorter sentences.
* “Reasoning over time instants, in addition to time intervals, is
also a distinctive feature of our work. For this reason, the
temporal representation is complemented by
instant (or point) based representations as well.” -> Reasoning on
timepoints is also performed in [Van Beek & R. Cohenand 1990] and in
[Solbrig et al. 2010].

Section 2.1
* “Description logics are a fragment of First Order Logic and
resolution-based approaches (i.e., reasoning methods for first order
logic) where initially employed for the required reasoning tasks.” -> seems redundant with respect to the beginning of this section. Rephrase the sentence or remove it.
* “To guarantee decidability,the rules are restricted to DL-safe
rules [16] that apply
only on named individuals in the ontology ABox.” -> be more precise
(either ontology or ABox)
Section 2.2
* “Choosing between a point or an interval-based representation is an important issue“ -> why is an important issue? I see that
there is a reference but be more explicit such that the text is
self-explaining since there there is no problem with the space

Section 2.4
* You list at the beginning of the subsection: “Temporal Description
logics (TDLs) [2], Concrete domains[15], Quadtuples [34],
Reification [19], Temporal RDF [8], Versioning [13], named graphs
[29] and 4D-fluents [32]” -> but then you describe only four of
them. Either remove the others or explain them and explain why you
are not considering them.
There is a difference between reification and n-ary relationshinp.
Please consider the definitions given in Rula et al 2012 and provide
the difference also in the paper.

Section 3.1
* “(or the equivalent property timeSliceOf)” and “(or the equivalent
property interval)”
-> it is not clear where did you define the equivalent properties.
you cannot introduce them in this way
* “tsTimeInterval (or the equivalent property interval) connects an
instance of class TimeSlice with an instance of class TimeInterval”
-> instance of class TimeInterval? so TimeInterval in Fig 5 is an
instance of class TimeInterval? recheck the sentence
* Second paragraph need to be re-written. It is not easy to be read,
difficult to follow.
* ”By allowing for qualitative relations the expressive power of the
representation increases.” -> rephrase
* the paragraph “Our approach demonstrates...” -> in which way do you
demonstrate that your approach enhance expressivity?

Section 4.1
* where did you define DOS. I understand that the meaning is During,
Overlaps and Starts but you should explain the first time what the
acronym means.
* page 10 second column you explain the symbols while those symbols
except intersection are used previously in the paper. You should
explain them only the first time they appear in the paper

Section 5
* “All experiments where run on a PC, with Intel Core CPU at 2.4
GHz, 6 GB RAM,
and Windows 7” -> were
* “Interval representations can be used for reasoning over 100
intervals, while qualitative representation combined with HermiT reasoner (representation I1 with HermiT, not presented in Figure 10)...” -> why representation I1 is not present in the Figure? Why I4
being a qualitative representation does not behave in the same way
as I1 since your comment is about qualitative representations.
* Figure 12, the x-axes you say Number of instances. Isn’t it the
number of intervals or the number of time points? How can you put
together intervals and points in a single graph?
* “For 100 intervals corresponding time is 2.03 seconds
respectively, clearly outperforming representations of Figure 10
(see 12)” -> who is outperforming who? make this sentence clearer.
You should refer to figure 12 and not just in brackets (see 12).
* Can you finish first with time intervals and then time points?
Then the order of figures and comments should be according to the
topic. so figure 12 goes before figure 11.
* page 16, second column last paragraph need to be revised

I tried to outline more minor comments to address the quality of
writing problems but again, this can only be considered an
incomplete list: *please* proof-read the paper more carefully before
submission. I strongly encourage the authors to improve the writing
throughout. I hope this second batch of detailed comments will help
in that direction.

Review #2
Anonymous submitted on 02/Sep/2015
Minor Revision
Review Comment:

The paper have been ameliorated but not all required corrections have been done, see below.
They MUST be corrected before acceptance.

"Building upon existing Semantic Web standards, tools and recommendations (like OWL 2, SWRL)"
SWRL is not a standard neither a recommendation but a Member Submission: this must be corrected (there are several occurrences in the paper)
The W3C recommendation for rules is RIF

This paragraph appears twice:

"Reasoning is implemented using SWRL rules and is capable of inferring temporal relations and detecting inconsistent assertions. The reasoning mechanism is an integral part of the ontology and is handled by standard reasoners (such as Pellet)."

"Description Logics (DLs) [3] are typically a decidable fragment of First Order Logic (FOL)"
not all DL are decidable


"SWRL is the language for specifying rules applying on Semantic Web ontologies"
SWRL is *a* language for specifying rules

"with the corresponding a point"
with the corresponding point

"Enforcing transitive properties is involved"
The formulation is not clear.

"It is fully compliant with existing Semantic Web standards and recommendations which increases its applicability. Being compatible with W3C recommendations the proposed framework"
SWRL is not a standard neither a recommendation.
The W3C recommendation for rules is RIF.

Review #3
By Aldo Gangemi submitted on 27/Feb/2016
Minor Revision
Review Comment:

The paper has improved since the first submission, but the central claims of the paper should be substantiated with a more orderly and explicit account of the work made, the evaluation performed, and a better writing.

Certain passages look like they are not well integrated with the rest of the paper, since there is no apparent dependency with either previous or following sentences, e.g.

p. 7
"In the original work by Welty and Fikes [32], the following restriction is imposed on timeslices: whenever two timeslices are related by means of a fluent prop- erty, their corresponding temporal intervals must be equal. However, no mechanism for enforcing this re- striction is provided. In this work, the following SWRL rule in conjunction with the reasoning mechanism of Section 4 imposes the required restriction"
p. 8
"Specifically, when a property is temporal, if the domain of property is ClassA and the range is ClassB (where domains and ranges can be composite class definitions or atomic concepts), then using the N-ary representation the domain becomes ClassA OR Event and the range ClassB OR Event. Compared to 4D-fluents, the disjunction of concepts appearing both in domain and ranges of properties limits specificity of references of the N-ary representation."

There is a lack of systematic comparison between the proposals: why both 4D and N-ary approaches are chosen, while no empirical comparison is performed (as on the contrary several contributions in the literature have attempted)? What is the authors' position?
The point/interval integration described in pages 8-9, and introduced as a central claim of the paper at p. 2, does not look novel, since it is already implemented in many temporal and foundational ontologies (Hobbs, OWL-Time, DOLCE, etc.), where both point representation, and interval representations are integrated exactly by providing start and end points to intervals. Where does the proposal improve over existing work?

Evaluation section
The evaluation is divided into theoretical and experimental, but only the second can be considered an evaluation (the first is an extension of the previous presentation and discussion). Furthermore, the evaluation concentrates on testing two reasoners (Pellet and HermiT) with various amounts of instances, time instants, or time intervals, which is useful, but it does not addresses the benefit of 4D or N-ary solutions as described by the authors, compared to alternative approaches.

p. 18
"Two alternative representations based on the 4D-fluents and the N-ary relations respectively are presented and evaluated."
ctually, the two are not compared in evaluation, just described.

Bibliography has been amended, but alphabetic order should be reintroduced

Use of commas is non-standard throughout the paper, and should be revised by a native speaker