# A Description Logic Primer

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2 semantics and use it to define the meaning ofsroiq ontologies. Many DLs can be obtained by omitting some features of SROIQ and in Section 5 we review some of the most important DLs obtained in this way. In particular, this includes various lightweight description logics that allow for particularly efficient reasoning. In Section 6 we discuss the relationship of DLs to the OWL Web Ontology Language. We conclude with pointers to further reading in Section Basic Building Blocks of DL Ontologies Description logics (DLs) provide means to model the relationships between entities in a domain of interest. In DLs there are three kinds of entities: concepts, roles and individual names. 1 Concepts represent sets of individuals, roles represent binary relations between the individuals, and individual names represent single individuals in the domain. Readers familiar with first-order logic will recognise these as unary predicates, binary predicates and constants. For example, an ontology modelling the domain of people and their family relationships might use concepts such Parent to represent the set of all parents and Female to represent the set of all female individuals, roles such as parentof to represent the (binary) relationship between parents and their children, and individual names such as julia and john to represent the individuals Julia and John. Unlike a database, a DL ontology does not fully describe a particular situation or state of the world ; rather it consists of a set of statements, called axioms, each of which must be true in the situation described. These axioms typically capture only partial knowledge about the situation that the ontology is describing, and there may be many different states of the world that are consistent with the ontology. Although, from the point of view of logic, there is no principal difference between different types of axioms, it is customary to separate them into three groups: assertional (ABox) axioms, terminological (TBox) axioms and relational (RBox) axioms Asserting Facts with ABox Axioms ABox axioms capture knowledge about named individuals, i.e., the concepts to which they belong and how they are related to each other. The most common ABox axioms are concept assertions such as Mother(julia), (1) which asserts that Julia is a mother or, more precisely, that the individual named julia is an instance of the concept Mother. Role assertions describe relations between named individuals. The assertion parentof(julia, john), (2) for example, states that Julia is a parent of John or, more precisely, that the individual named julia is in the relation that is represented by parentof to the individual named john. 1 In OWL concepts and roles are respectively known as classes and properties; see Section 6. 2

3 The previous sentence shows that it can be rather cumbersome to explicitly point out that the relationships expressed by an axiom are really relationships between the individuals, sets and relations that are represented by the respective individual names, concepts and roles. Assuming that this subtle distinction between syntactic identifiers and semantic entities is understood, we will thus often adopt a more sloppy and readable formulation. Section 4 below explains the underlying semantics with greater precision. Although it is intuitively clear that Julia and John are different individuals, this fact does not logically follow from what we have stated so far. DLs do not make the unique name assumption, so different names might refer to the same individual unless explicitly stated otherwise. The individual inequality assertion julia john (3) is used to assert that Julia and John are actually different individuals. On the other hand, an individual equality assertion, such as john johnny, (4) states that two different names are known to refer to the same individual. Such situations can arise, for example, when combining knowledge about the same domain from several different sources, a task that is known as ontology alignment Expressing Terminological Knowledge with TBox Axioms TBox axioms describe relationships between concepts. For example, the fact that all mothers are parents is expressed by the concept inclusion Mother Parent, (5) in which case we say that the concept Mother is subsumed by the concept Parent. Such knowledge can be used to infer further facts about individuals. For example, (1) and (5) together imply that Julia is a parent. Concept equivalence asserts that two concepts have the same instances, as in Person Human. (6) While synonyms are an obvious example of equivalent concepts, in practice one more often uses concept equivalence to give a name to complex expressions as introduced in Section 2.1 below. Furthermore, such additional concept expressions can be combined with equivalence and inclusion to describe more complex situations such as the disjointness of concepts, which asserts that two concepts do not share any instances Modelling Relationships between Roles with RBox Axioms RBox axioms refer to properties of roles. As for concepts, DLs support role inclusion and role equivalence axioms. For example, the inclusion parentof ancestorof (7) 3

4 states that parentof is a subrole of ancestorof, i.e., every pair of individuals related by parentof is also related by ancestorof. Thus (2) and (7) together imply that Julia is an ancestor of John. In role inclusion axioms, role composition can be used to describe roles such as uncleof. Intuitively, if Charles is a brother of Julia and Julia is a parent of John, then Charles is an uncle of John. This kind of relationship between the roles brotherof, parentof and uncleof is captured by the complex role inclusion axiom brotherof parentof uncleof. (8) Note that role composition can only appear on the left-hand side of complex role inclusions. Furthermore, in order to retain decidability of reasoning (see the end of Section 4 for a discussion on decidability), complex role inclusions are governed by additional structural restrictions that specify whether or not a collection of such axioms can be used together in one ontology. Nobody can be both a parent and a child of the same individual, so the two roles parentof and childof are disjoint. In DLs we can write disjoint roles as follows: Disjoint(parentOf, childof). (9) Further RBox axioms include role characteristics such as reflexivity, symmetry and transitivity of roles. These are closely related to a number of other DL features and we will discuss them again in more detail in Section Constructors for Concepts and Roles The basic types of axioms introduced in Section 1 are rather limited for accurate modelling. To describe more complex situations, DLs allow new concepts and roles to be built using a variety of different constructors. We distinguish concept and role constructors depending on whether concept or role expressions are constructed. In the case of concepts, one can further separate basic Boolean constructors, role restrictions and nominals/enumerations. At the end of this section, we revisit the additional kinds of RBox axioms that have been omitted in Section Boolean Concept Constructors Boolean concept constructors provide basic Boolean operations that are closely related to the familiar operations of intersection, union and complement of sets, or to conjunction, disjunction and negation of logical expressions. For example, concept inclusions allow us to state that all mothers are female and that all mothers are parents, but what we really mean is that mothers are exactly the female parents. DLs support such statements by allowing us to form complex concepts such as the intersection (also called conjunction) Female Parent, (10) 4

5 which represents the set of individuals that are both female and parents. A complex concept can be used in axioms in exactly the same way as an atomic concept, e.g., in the equivalence Mother Female Parent. Union (also called disjunction) is the dual of intersection. For example, the concept Father Mother (11) describes those individuals that are either fathers or mothers. Again, it can be used in an axiom such as Parent Father Mother, which states that a parent is either a father or a mother (and vice versa). Sometimes we are interested in individuals that do not belong to a certain concept, e.g., in women who are not married. These could be described by the complex concept Female Married, (12) where the complement (also called negation) Married represents the set of all individuals that are not married. It is sometimes useful to be able to make a statement about every individual, e.g., to say that everybody is either male or female. This can be accomplished by the axiom Male Female, (13) where the top concept is a special concept with every individual as an instance; it can be viewed as an abbreviation for C C for an arbitrary concept C. Note that this modelling is rather coarse as it presupposes that every individual has a gender, which may not be reasonable for instances of a concept such as Computer. We will see more useful applications for later on. To express that, for the purposes of our modelling, nobody can be both a male and a female at the same time, we can declare the set of male and the set of female individuals to be disjoint. While ontology languages like OWL provide a basic constructor for disjointness, it is naturally captured in DLs with the axiom Male Female, (14) where the bottom concept is the dual of, that is the special concept with no individuals as instances; it can be seen as an abbreviation for C C for an arbitrary concept C. The above axiom thus says that the intersection of the two concepts is empty Role Restrictions So far we have seen how to use TBox and RBox axioms to express relationships between concepts and roles, respectively. The most interesting feature of DLs, however, is their ability to form statements that link concepts and roles together. For example, there is an obvious relationship between the concept Parent and the role parentof, namely, a parent is someone who is a parent of at least one individual. In DLs, this relationship can be captured by the concept equivalence Parent parentof., (15) 5

6 where the existential restriction parentof. is a complex concept that describes the set of individuals that are parents of at least one individual (instance of ). Similarly, the concept parentof.female describes those individuals that are parents of at least one female individual, i.e., those that have a daughter. To represent the set of individuals all of whose children are female, we use the universal restriction parentof.female. (16) It is a common error to forget that (16) also includes those individuals that have no children at all. More accurately (and less naturally), the axiom can be said to describe the set of all individuals that have no children other than female ones, i.e., that have no children that are not female. Following this wording, the concept (16) could indeed be equivalently expressed as parentof. Female. If this meaning is not intended, one can describe the individuals who have at least one child and with all their children being female by the concept ( parentof. ) ( parentof.female). Existential and universal restrictions are useful in combination with the top concept for expressing domain and range restrictions on roles; that is, restrictions on the kinds of individual that can be in the domain and range of a given role. To restrict the domain of sonof to male individuals we can use the axiom and to restrict its range to parents we can write sonof. Male, (17) sonof.parent. (18) In combination with the assertion sonof(john, julia), these axioms would then allow us to deduce that John is male and Julia is a parent. It is interesting to note how this behaviour contrasts with the meaning of constraints in databases. Constraints would also allow us to state, e.g., that all sons must be male. However, given only the fact that John is a son of Julia, such a constraint would simply be violated (leading to an error) rather than implying that John is male. Mistaking DL axioms for constraints is a very common source of modelling errors. Number restrictions allow us to restrict the number of individuals that can be reached via a given role. For example, we can form the at-least restriction 2 childof.parent (19) to describe the set of individuals that are children of at least two parents, and the at-most restriction 2 childof.parent (20) for those that are children of at most two parents. The axiom Person 2 childof.parent 2 childof.parent then states that every person is a child of exactly two parents. 6

7 Finally, local reflexivity can be used to describe the set of individuals that are related to themselves via a given role. For example, the set of individuals that talk to themselves is described by the concept 2.3. Nominals talksto.self. (21) As well as defining concepts in terms of other concepts (and roles), it may also be useful to define a concept by simply enumerating its instances. For example, we might define the concept Beatle by enumerating its instances: john, paul, george, and ringo. Enumerations are not supported natively in DLs, but they can be simulated in DLs using nominals. A nominal is a concept that has exactly one instance. For example,{john} is the concept whose only instance is (the individual represented by) john. Combining nominals with union, the enumeration in our example could be expressed as Beatle {john} {paul} {george} {ringo}. (22) It is interesting to note that, using nominals, a concept assertion Mother(julia) can be turned into a concept inclusion{julia} Mother and a role assertion parentof(julia, john) into a concept inclusion{julia} parentof.{john}. This illustrates that the distinction between ABox and TBox does not have a deeper logical meaning Role Constructors In contrast to the variety of concept constructors, DLs provide only few constructors for forming complex roles. In practice, inverse roles are the most important such constructor. Intuitively, the relationship between the roles parentof and childof is that, for example, if Julia is a parent of John, then John is a child of Julia and vice versa. More formally, parenfof is the inverse of childof, which in DLs can be expressed by the equivalence parentof childof, (23) where the complex role childof represents the inverse of childof. In analogy to the top concept, DLs also provide the universal role, represented by U, which always relates all pairs of individuals. It typically plays a minor role in modelling, 2 but it establishes symmetry between roles and concepts w.r.t. a top element. Similarly, an empty role that corresponds to the bottom concept is also available in OWL but has rarely been introduced as a constructor in DLs; however, we can define any role R to be empty using the axiom R. ( all things do not relate to anything through R ). Interestingly, the universal role cannot be defined by TBox axioms using the constructors introduced above, and in particular universal role restrictions cannot express that a role is universal. 2 Although there are a few interesting things that could be expressed with U, such as concept products [16], tool support is rarely sufficient for using this feature in practice. 7

8 2.5. More RBox Axioms: Role Characteristics In Section 1.3 we introduced three forms of RBox axioms: role inclusions, role equivalences and role disjointness. OWL provides a variety of others, namely role transitivity, symmetry, asymmetry, reflexivity and irreflexivity. These are sometimes considered as basic axiom types in DLs as well, using some suggestive notation such as Trans(ancestorOf) to express that the role ancestorof is transitive. However, such axioms are just syntactic sugar; all role characteristics can be expressed using the features of DLs that we have already introduced. Transitivity is a special form of complex role inclusion. For example, transitivity of ancestorof can be captured by the axiom ancestorof ancestorof ancestorof. A role is symmetric if it is equivalent to its own inverse, e.g., marriedto marriedto, and it is asymmetric if it is disjoint from its own inverse, as in Disjoint(parentOf, parentof ). If desired, global reflexivity can be expressed by imposing local reflexivity on the top concept as in knows.self. A role is irreflexive if it is never locally reflexive, as in the case of marriedto.self. 3. The Description LogicSROIQ In this section, we summarise the various features that have been introduced informally above to provide a comprehensive definition of DL syntax. Doing so yields the description logic called SROIQ, which is one of the most expressive DLs commonly considered today. It also largely agrees in expressivity with the ontology language OWL 2 DL, though there are still some differences as explained in Section 6. Formally, every DL ontology is based on three finite sets of signature symbols: a set N I of individual names, a set N C of concept names and a set N R of role names. Usually these sets are assumed to be fixed for some application and are therefore not mentioned explicitly. Now the set ofsroiq role expressions R (over this signature) is defined by the following grammar: R U N R N R where U is the universal role (Section 2.4). Based on this, the set ofsroiq concept expressions C is defined as: C N C (C C) (C C) C R.C R.C n R.C n R.C R.Self {N I } where n is a non-negative integer. As usual, expressions like (C C) represent any expression of the form (C D) with C, D C. It is common to omit parentheses if this cannot lead to confusion with expressions of different semantics. For example, parentheses do not matter for A B C whereas the expressions A B C and R.A B are ambiguous. Using the above sets of individual names, roles and concepts, the axioms of SROIQ can be defined to be of the following basic forms: 8

9 ABox: C(N I ) R(N I, N I ) N I N I N I N I TBox: C C C C RBox: R R R R R R R Disjoint(R, R) with the intuitive meanings as explained in Section 1 and 2. Roughly speaking, asroiq ontology (or knowledge base) is simply a set of such axioms. To ensure the existence of reasoning algorithms that are correct and terminating, however, additional syntactic restrictions must be imposed on ontologies. These restrictions refer not to single axioms but to the structure of the ontology as a whole, hence they are called structural restrictions. The two such conditions relevant for SROIQ are based on the notions of simplicity and regularity. Notably, both are automatically satisfied for ontologies that do not contain complex role inclusion axioms. A role R in an ontologyo is called non-simple if some complex role inclusion axiom (i.e., one that uses role composition ) inoimplies instances of R; otherwise it is called simple. A more precise definition of the non-simple role expressions of the ontology O is given by the following rules: if O contains an axiom S T R, then R is non-simple, if R is non-simple, then its inverse R is also non-simple, 3 if R is non-simple andocontains any of the axioms R S, S R or R S, then S is also non-simple. All other roles are called simple. 4 Now for asroiq ontology it is required that the following axioms and concepts contain simple roles only: Restricted axioms: Disjoint(R, R) Restricted concept expressions: R.Self n R.C n R.C. The other structural restriction that is relevant for SROIQ is called regularity and is concerned with RBox axioms only. Roughly speaking, the restriction ensures that cyclic dependencies between complex role inclusion axioms occur only in a limited form. For details, please see the pointers given in Section 7. For the introductory treatment in this paper, it suffices to note that regularity, just like simplicity, is a property of the ontology as a whole that cannot be checked for each axiom individually. An important practical consequence is that the union of two regular ontologies may no longer be regular. This must be taken into account when merging ontologies in practice. 4. Description Logic Semantics The formal meaning of DL axioms is given by their model-theoretic semantics. In particular, the semantics specifies what the logical consequences of an ontology are. The formal semantics is therefore the main guideline for every tool that computes logical consequences of DL ontologies, and a basic understanding of its working is vital to make 3 If R=S already is an inverse role, then R should be read as S. We do not allow expressions like S. 4 Whether the universal role U is simple or not is a matter of preference that does not affect the computational properties of the logic [17]. However, the universal role in OWL 2 is considered non-simple. 9

10 reasonable modelling choices and to comprehend the results given by software applications. Luckily, the semantics of description logics is not difficult to understand provided that some common misconceptions are avoided. Intuitively speaking, an ontology describes a particular situation in a given domain of discourse. For example, the axioms in Sections 1 and 2 describe a particular situation in the families and relationships domain. However, ontologies usually cannot fully specify the situation that they describe. On the one hand, there is no formal relationship between the symbols we use and the objects that they represent: the individual name julia, for example, is just a syntactic identifier with no intrinsic meaning. Indeed, the intended meaning of the identifiers in our ontologies has no influence on their formal semantics: what we know about them stems only from the ontological axioms. On the other hand, the axioms in an ontology typically do not provide complete information. For example, (3) and (4) in Section 1.1 state that some individuals are equal and that others are unequal, but in many other cases this information might be left unspecified. Description logics have been designed to deal with such incomplete information. Rather than making default assumptions in order to fully specify one particular interpretation for each ontology, the DL semantics generally considers all the possible situations (i.e., states of the world) where the axioms of an ontology would hold (we also say: where the axioms are satisfied). This characteristic is sometimes called the Open World Assumption since it keeps unspecified information open. 5 A logical consequence of an ontology is an axiom that holds in all interpretations that satisfy the ontology, i.e., something that is true in all conceivable states of the world that agree with what is said in the ontology. The more axioms an ontology contains, the more specific are the constraints that it imposes on possible interpretations, and the fewer interpretations exist that satisfy all of the axioms. Conversely, if fewer interpretations satisfy an ontology, then more axioms hold in all of them, and more logical consequences follow from the ontology. The previous two sentences imply that the semantics of description logics is monotonic: additional axioms always lead to additional consequences, or, more informally, the more knowledge we feed into a DL system the more results it returns. An extreme case is when an ontology is not satisfied in any interpretation. The ontology is then called unsatisfiable or inconsistent. In this case every axiom holds vacuously in all of the (zero) interpretations that satisfy the ontology. Such an ontology is clearly of no utility, and avoiding inconsistency (and checking for it in the first place) is therefore an important task during modelling. We have outlined above the most important ideas of DL semantics. What remains to be done is to define what we really mean by an interpretation and which conditions must hold for particular axioms to be satisfied by an interpretation. For this, we closely follow the intuitive ideas established above: an interpretationiconsists of a set I called the domain ofiand an interpretation function I that maps each atomic concept A to a set A I I, each atomic role R to a binary relation R I I I, and each individual name a to an element a I I. The interpretation of complex concepts and roles follows from the interpretation of the basic entities. Table 1 shows how to obtain the semantics of each compound expression from the semantics of its parts. By R I -successor of x we mean any individual y such that x, y R I. The definition should confirm the intuitive 5 A Closed World Assumption closes the interpretation by assuming that every fact not explicitly stated to be true is actually false. Both terms are not formally specified and rather outline the general flavour of a semantics than any particular definition. 10

11 Individuals: Table 1. Syntax and semantics of SROIQ constructors Syntax Semantics individual name a a I Roles: atomic role R R I inverse role R { x, y y, x R I } universal role U I I Concepts: atomic concept A A I intersection C D C I D I union C D C I D I complement C I \ C I top concept I bottom concept existential restriction R.C {x some R I -successor of x is in C I } universal restriction R.C {x all R I -successors of x are in C I } at-least restriction n R.C {x at least n R I -successors of x are in C I } at-most restriction n R.C {x at most n R I -successors of x are in C I } local reflexivity R.Self {x x, x R I } nominal {a} {a I } where a, b N I are individual names, A N C is a concept name, C, D Care concepts, R Ris a role Table 2. Syntax and semantics of SROIQ axioms Syntax Semantics ABox: concept assertion C(a) a I C I role assertion R(a, b) a I, b I R I individual equality a b a I = b I individual inequality a b a I b I TBox: concept inclusion C D C I D I concept equivalence C D C I = D I RBox: role inclusion R S R I S I role equivalence R S R I = S I complex role inclusion R 1 R 2 S R I 1 RI 2 SI role disjointness Disjoint(R, S ) R I S I = explanations given for each case in Section 2. For example, the semantics of Female Parent is indeed the intersection of the semantics of Female and Parent. Since an interpretation I fixes the meaning of all entities, we can unambiguously say for each axiom whether it holds inior not. An axiomαholds ini(we also sayi satisfiesαand writei =α) if the corresponding condition in Table 2 is met. Again, these definitions fully agree with the intuitive explanations given in Section 1. If all axioms in an ontologyohold ini(i.e., ifisatisfieso, writteni =O), theniis a model 11

12 ofo. Thus a model is an abstraction of a state of the world that satisfies all axioms in the ontology. An ontology is consistent if it has at least one model. An axiomαis a consequence of an ontology O (or O entails α, written O = α) if α holds in every model ofo. In particular, an inconsistent ontology entails every axiom. A noteworthy consequence of this semantics is the meaning of individual names in DL ontologies. We already remarked that DLs do not usually make the Unique Name Assumption, and indeed our formal definition allows two individual names to be interpreted as the same individual (element of the domain). Possibly even more important is the fact that the domain of an interpretation is allowed to contain many individuals that are not represented by any individual name. A common confusion in modelling arises from the implicit assumption that interpretations must only contain individuals that are represented by individual names (such individuals are also called named individuals). For example, one could wrongly assume the ontology consisting of the axioms parentof(julia, john) manychildren(julia) manychildren 3 parentof. to be inconsistent since it requires Julia to have at least 3 children when only one (John) is given. However, there are many conceivable models where Julia does have three children, even though only one of the children is explicitly named. A significant number of modelling errors can be traced back to similar misconceptions that are easy to prevent if the general open world assumption of DLs is kept in mind. Another point to note is that the above specification of the semantics does not provide any hint as to how to compute the relevant entailments in practical software tools. There are infinitely many possible interpretations, each of which may have an infinite domain (in fact there are some ontologies that are satisfied only by interpretations with infinite domains). Therefore it is impossible to test all interpretations to see if they model a given ontology, and impossible to test all models of an ontology to see if they entail a given axiom. Rather, one has to devise deduction procedures and prove their correctness with respect to the above specification. The interplay of certain expressive features can make reasoning algorithms more complicated and in some cases it can even be shown that no correct and terminating algorithm exists at all (i.e., that reasoning is undecidable). For our purposes it suffices to know that entailment of axioms is decidable for SROIQ (with the structural restrictions explained in Section 3) and that a number of free and commercial tools are available. Such tools are typically optimised for more specific reasoning problems, such as consistency checking, the entailment of concept subsumptions (subsumption checking) or of concept assertions (instance checking). Many of these standard inferencing problems can be expressed in terms of each other, so they can be handled by very similar reasoning algorithms. 5. Important Fragments ofsroiq Many different description logics have been introduced in the literature. Typically, they can be characterised by the types of constructors and axioms that they allow, which are often a subset of the constructors in SROIQ. For example, the description logic ALC is the fragment ofsroiq that allows no RBox axioms and only,,, and as its concept constructors. The extension of ALC with transitive roles is traditionally denoted 12

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