From: AAAI-94 Proceedings. Copyright © 1994, AAAI (www.aaai.org). All rights reserved. Formalizing Ontological Commitments Nicola Guarino Massimiliano Carrara Pierdaniele Giaretta LADSEB-CNR, National Research Council, Viale Ungheria, 43a Institute of History of Philosophy, Corso Stati Uniti, 4 I-37046 Minerbe (VR) University of Padova, I-35127 Padova, Italy Italy Piazza Capitaniato, 3 [email protected] I-35100 Padova, Italy Abstract purposes, since we want to include in the commitment Formalizing the ontological commitment of a logical lan- some basic assumptions and distinctions presupposed by guage means offering a way to specify the intended meaning the theory. of its vocabulary by constraining the set of its models, giv- In the AI community, the above position is at the basis ing explicit information about the intended nature of the of current projects for knowledge sharing and reuse modelling primitives and their a priori relationships. We (Neches et al. 1991). In the knowledge acquisition litera- present here a formal definition of ontological commitment ture, the notion of ontological commitment has been intro- which aims to capture the very basic ontological assump- duced by Gruber (1993-1994) as an agreement to use a tions about the intended domain, related to issues such as sli,ared vocabukary specification: such a specification is a identity and internal structure. To tackle such issues, a set of terminological ‘axioms, <and ontological commitment modal framework endowed with mereo-topological primi- amounts to syntactical consistency with such axioms. This tives has been adopted. The paper is mostly based on a re- syntactical notion does not fit our intuitions, since it seems visitation of philosophical (and linguistic) literature in the natural to allow two different vocabularies (using English perspective of knowledge representation. OI- Italian words, for instance) to share the same ontology. In other words, the notion of ontological commitment ILIntroduction should be a semantic one, not a syntactic one. A semantic notion which gets closer to our purposes is First order logic is notoriously neutral with respect to that of conc~~?tualization, defined in (Gcnesereth & Nilsson ontological choices: when a logical language is used with 1987) as a triple consisting of a domain, a set of functions the purpose of modelling a particular aspect of reality, the (which we ignore for our purposes) and a set of relations on set M of all its models is usucally much larger than the set that domain. For instance (pp. 9-12), the triple <{a, b, c}, Mi of the intended ones, which describe only those states 0, i on, shove, tutYe}> is a conceptualization of a situation of affairs which are compatible with some underlying onto- describing some block on a table. The authors note Zogicnl commitment. Such a commitment is usually implied however that rimes of ob.jects and relations refer to purely by the vocabuhy used, i.e. by the symbols chosen as con- extensional entities. They describe therefore a particulol stants and predicates: we sistematically use natural lan- state of affairs, without telling us anything about other guage words within our theories, relying on them to make possible states of ‘affairs. On the other hand, the intended our statements readable and to convey meanings not ex- meaning implied by the nrames chosen for the relevant plicitly stated. However, since words are often vague and relations constmints Nil the possible states of ,affairs. ambiguous in natural language, it may be important to con- In conclusion, ontological commitment cannot be UII- strain their semantics in order to gu‘arantee a consistent in- derstood as “an explicit specification of a conceptualiza- terpretation. This is unavoidable, in our opinion, if we want tion” (Gruber 1993, p. 199), at least in the technical sense to share theories across different domains (Neches et al. of the latter term. Rather, an ontological commitment 1991, Gruber 1993). should capture and constrain a set of conceptualizations. In the pliilosopliical literature, the notion of ontological Formalizing the ontological commitment of a logical km- commitment was first introduced by Quine (1961). Accor- bouage means offering a way to specify the intended mean- ding to him, a theory is ontologically commited to the enti- ing of its vocabukary by constraining the set of its models, ties which it quantifies over: “to be is to be the value of a oiving explicit information about the intended nuture of the variable”. Such criterion was further refined by Church bodelling primitives used and their CIpriori relationships. (1958) and Alston (1958), and finally modified by Searle In this sense, an ontological commitment is a mapping be- (1969) in order to defend his argument that the ontological tween a language and something which can be called an commitment of a theory simply coincides whit11 what it as- on tology. serts. We reject the latter position, holding that non- Consider a first-order language L, and a particular the- equivalent theories can share the scame commitment. On the ory T of L. A possible way to formalize the ontological other hand, Quine’s proposal seems to be too we<ak for our commitment of L is by specifying the set Mi of its intended 560 Knowledge Bases models’ by means of a suitable theory which uses the same ous to the KR tradition. In section 2 we give an example language L. The only purpose of such a theory is to specify intended to motivate the kinds of distinctions we want to (or at least approximate) the meaning of the vocabulary account for within an ontological commitment. After the used. Such a theory should be kept separated from theories presentation of our formal framework in section 3, we which use the same vocabulary making assertions about show in section 4 how some fundamental ontological particular states of affairs. Current approaches to the properties of predicates can be expressed within that problem of knowledge sharing (Neches et al. 1991, Gruber framework. A detailed analysis of meta-level ontological 1993-1994) are along this line: a common (sub)theory categories of unary predicates has been carried out in called terminology describes the shared ontology, while (Guarino, Carrara & Giaretta 1994). In this paper, we task dependent knowledge is specified by separate theories. underline the necessity to adopt such distinctions as an The approach described above is not satisfactory, how- uneludible part of any formal attempt to capture ontological ever, if our purpose is the formalization of the ontological commitment. We focus in particular on unary predicates, commitment of an arbitrary language L. The reason is that arguing that - in order to specify their intended meaning - nothing guarantees us that the vocabulary of L is adequate a first, fundamental choice regards the distinction between to express the ontological constraints we are interested in: so-called sortal and non-sortal predicates. if we want to capture the a priori structure of individuals Such a distinction was originally introduced by Locke we need enough granularity to be able to speak of their in- and discussed in (Strawson 1959) and (Wiggins 1980). ternal constitution, while to capture the nature of individu- According to Strawson, a sortal predicate (like apple) “sup- als and relations we also need suitable primitive categories. plies a principle for distinguishing and counting individual For instance, nothing is said in the example mentioned particulars which it collects”, while a non-sortal predicate above about the nature of the domain: are a, b, and c (like red) “supplies such a principle only for particulars physical objects or spatial regions? already distinguished, or distinguishable, in accordance A further limitation to the formalization of the ontologi- with some antecedent principle or method”. This dis- cal commitment of a language L by means of a first-order tinction is (roughly) reflected in natural language by the theory of L comes from the fact that ontology, being fact that the former terms are common nouns, while the lat- knowledge about a priori structure of reality, is intimately ter are adjectives and verbs. It is implicitly present in the related to a notion of modality: choosing a particular in- KR literature, where sortal predicates are usually called tended model for a logical theory implies making implicit “concepts”, while characterising predicates are called assumptions about other models compatible with the cho- “properties”. Within current KR formalisms, however, the sen one. In other words, there are constraints among possi- difference between the two is only based on heuristic con- ble models which reflect some important aspects of reality: siderations, and nothing in the semantics of a concept for- for instance, models describing the temporal evolution of a bids it from being treated like any other unary predicate. situation should share the same interpretation for the indi- The notion of well-founded ontological commitment is vidual constants used in the description of that situation. introduced in section 5, with the purpose of offering some We present in this paper a formal notion of the ontologi- concrete methodological guidelines to the current practice cal commitment of a language L, expressed by means of a of knowledge engineering. theory T’ which uses a language L’ richer than L. Such a language extends both the logical symbols and the vocabulary of L by adding modal operators, mereo- topological relations and basic domain categories. Since the reliminary Example only purpose of T’ is to specify the intended use of L, it is Suppose we want to state that a red apple exists. In stan- not necessary to replace T with a larger theory T”=T u T dard first-order logic, it is a simple matter to write down of L’: for instance, a particular ontological property of a something like 3x.(Ax A Rx)~.