Reconciling OWL and Non-Monotonic Rules for the Semantic Web

Reconciling OWL and Non-monotonic Rules for the Semantic Web Matthias Knorr1 and Pascal Hitzler2 and Frederick Maier3 Abstract. We propose a description logic extending SROIQ In principle, the second rift cannot be completely overcome. Nev- (the description logic underlying OWL 2 DL) and at the same time ertheless, several decidable combinations of OWL and rules have encompassing some of the most prominent monotonic and non- been proposed in the literature, usually resulting in a hybrid formal- monotonic rule languages, in particular Datalog extended with the ism mixing the syntax and sometimes even the semantics of rules and answer set semantics. Our proposal could be considered a substan- description logics (see the survey sections of [18, 17]). The most re- tial contribution towards fulfilling the quest for a unifying logic for cent proposal [20] rests on the introduction of a new syntax construct the Semantic Web. As a case in point, two non-monotonic extensions to description logics, called nominal schemas, which results in a de- of description logics considered to be of distinct expressiveness until scription logic which seamlessly—syntactically and semantically— now are covered in our proposal. In contrast to earlier such propos- incorporates binary DL-safe Datalog [24] (and as we will see in Sec- als, our language has the “look and feel” of a description logic and tion 3.1, even incorporates unrestricted DL-safe Datalog). avoids hybrid or first-order syntaxes. While we would argue that the introduction of nominal schemas constitutes a major advance towards a reconciliation of OWL and rules, expanding OWL with nominal schemas by itself does nothing 1 Introduction to resolve the first of the rifts mentioned above. And so the approach described in [20] can only be viewed as a partial reconciliation. The landscape of ontology languages for the Semantic Web is di- The main purpose of the present paper is to build upon the work of verse and controversial [11]. In terms of expressive ontology repre- [20], addressing that first rift. We show that nominal schemas allow sentation formalisms, this is most clearly reflected by the two W3C not only for a concise reconciliation of OWL and Datalog, but also standards RIF [15] and OWL [10], where the former—the Rule In- that the integration can in fact be lifted to cover established closed terchange Format—is based on rules in the wider sense including world formalisms on both the OWL and the rules side. More pre- Datalog and logic programming [12], and the latter—the Web Ontol- cisely, we endow SROIQ, the description logic underlying OWL 2 ogy Language—is based on description logics [1]. In terms of both DL, with both nominal schemas and a generalized semantics based academic research and industrial development, these two formalisms on the logic of minimal knowledge and negation as failure (MKNF) cater to almost disjoint subcommunities. [7, 22]. The latter, and thus also the extension of SROIQ based on While the different paradigms often focus on different perspec- it, is non-monotonic and captures both open and closed world mod- tives and needs, the field and its applications would as a whole ben- eling. We show that it in fact encompasses major ontology modeling efit if a certain coherence were retained. This coherence could be paradigms, including (trivially) OWL 2 DL and its tractable frag- achieved through the use of a unifying logic, one which reconciles ments [10] but also unrestricted DL-safe Datalog, MKNF-extended the diverging paradigms of the Semantic Web stack.4 Indeed, several ALC [7], hybrid MKNF [23], description logics with defaults [2], proposals have been made towards creating such a unified logic, but and the answer set semantics for Datalog with negation [9]. This the quest remains largely unfulfilled. means it also covers various ways of expressing the closure of con- Two major rifts have been identified which need to be overcome cepts and roles, and also of expressing integrity constraints. for a reconciliation to occur. This is particularly true in the case of The plan of the paper is as follows. In Section 2 we in- OWL and rule languages. The first rift is due to a fundamental con- troduce the syntax and semantics of our new logic, called ceptual difference in how the paradigms deal with unknown informa- SROIQV(Bs; ×)K , and we show that decidable reasoning in tion. While OWL adheres to the so-called Open World Assumption NF a sufficiently large fragment of it can be realized. In Section 3, we (OWA), and thus treats unknown information indeed as unknown, show how SROIQV(Bs; ×)K encompasses many of the well- rules and logic programming adhere to the so-called Closed World NF known languages and proposals related to the integration of OWL, Assumption (CWA) in which unknown information defaults to false, rules, and non-monotonicity. Section 4 concludes with brief remarks i.e., the knowledge available is thought to be a complete encoding of on related work and a discussion of future work. the domain of interest. The second rift is caused by the decision to An extended version of the paper (including proofs, exam- design OWL as a decidable language: Naive combinations (e.g., in ples, and many more details in Section 2.3) is available at first-order predicate logic) of rule languages (such as Datalog) and http://centria.di.fct.unl.pt/∼mknorr/resources/KHM-ECAI12ex.pdf. OWL are undecidable, and thus violate this design decision. 1 s CENTRIA, Universidade Nova de Lisboa 2 MKNF DL SROIQV(B ; ×)KNF 2 Kno.e.sis Center, Wright State University s 3 Aston Business School, Aston University Our work is based on the description logic (DL) SROIQV(B ; ×). 4 http://www.w3.org/2007/03/layerCake.png It extends SROIQ [11, 13] with concept products [19] and Boolean constructors over simple roles [25]. Importantly, it also incorporates Additionally SROIQ admits RBox axioms that directly express nominal schemas [20]. These represent variable nominals that can the empty role, role disjointness, asymmetry, reflexivity, irreflexibil- only bind to known individuals. It has been shown that none of these ity, symmetry, and transitivity. As shown in [20], all these can be extensions affect the worst case complexity of reasoning in SROIQ expressed in SROIQV(Bs; ×) anyway, so we omit them here. In [20, 19]. We refer to [1, 11] for a detailed account on DLs in general contrast to [20], we explicitly allow the usage of complex concepts and to [20] for SROIQV(Bs; ×) in particular. and roles in the ABox to simplify the presentation in the next sec- Following the work in [7], where the DL ALC is augmented with tion. This difference is only syntactic, as such expressions can easily two modal operators K and A, we define such an extension for be reduced by introducing new concept and role names. SROIQV(Bs; ×). The modal operator K is interpreted in terms of minimal knowledge, while A is interpreted as autoepistemic as- 2.2 Semantics sumption and corresponds to :not, i.e., the classical negation of the s negation as failure operator not used in [22] instead of A. The ex- The semantics of SROIQV(B ; ×)KNF is a generalization of the s s tension to SROIQV(B ; ×) is non-trivial in so far as this DL is sig- semantics of SROIQV(B ; ×) [20] and that of ALCKNF [7]. We nificantly more expressive than ALC and, in particular, the usage of first recall two basic notions from [20]. modal operators in role expressions is considerably more advanced. An interpretation I = (∆I ; ·I ) consists of a domain ∆I 6= ; and I We introduce the syntax and semantics of our proposed language, a function · that maps elements in NI , NC , and NR to elements, s I I called SROIQV(B ; ×)KNF in the nomenclature introduced in sets, and relations of ∆ respectively, i.e., for a 2 NI , a = d 2 I I I I I I [7], and we provide some remarks on a reasoning procedure. ∆ , for A 2 NC , A ⊆ ∆ , and, for V 2 NR, V ⊆ ∆ × ∆ . A variable assignment Z for an interpretation I is a function Z : I I 2.1 Syntax NV ! ∆ such that, for each v 2 NV , Z(v) = a for some a 2 NI . We consider a signature Σ = hNI ;NC ;NR;NV i where NI , NC , As is common in MKNF-related semantics used to combine DLs NR, and NV are pairwise disjoint and finite sets of individual names, with non-monotonic reasoning (see [7, 14, 16, 23]), specific restric- concept names, role names, and variables. Role names are divided tions on interpretations are introduced to ensure that certain unin- s into disjoint sets of simple role names NR and non-simple role names tended logical consequences can be avoided (see, e.g., [23]). We n NR. In the following, we assume that Σ has been fixed. We define adapt the standard name assumption from [23]. concepts and roles in SROIQV(Bs; ×)K as follows. NF Definition 3 An interpretation I (over Σ to which ≈ is added) em- s Definition 1 The set of SROIQV(B ; ×)KNF concepts C and ploys the standard name assumption if s s n (simple/non-simple) SROIQV(B ; ×)KNF roles R (R /R ) are ∗ (1) NI extends NI with a countably infinite set of individuals that defined by the following grammar. I ∗ cannot be used in variable assignments, and ∆ = NI ; s s s − s s s s s (2) for each i in N ∗, iI = i; and R ::=NR j (NR) j U j NC × NC j :R j R u R j R t R j I (3) equality ≈ is interpreted in I as a congruence relation – that is, ≈ KRs j ARs is reflexive, symmetric, transitive, and allows for the replacement n n n − n n R ::=NR j (NR) j U j NC × NC j KR j AR of equals by equals [8].

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