An Empirically Based Approach Towards a System of Semantic Features

A major problem in a~chine translation is the semantic description of lexical units which should be based on a semantic system that is bot]l coherent and operationalized to the greatest possible degree° This is to guarantee consistency between lexical unii~ coded by lexicogral~lers. This article introduces a generating device for achieving wellformed semantic feature expressions. I. T ntention and procedure Empirical ~xk with the verbs of the ES~rpus as well as experience in theoretical semantics, and last but not least, the consulting of the semantic feature inventories of other machine translation systems (MII.~AL, JAPAN, SYSTRAN, SUSY) l~ns resulted in the necessity of an elaboration of the proposal for semantic features made in EIB-3 (EUROTRA LINGUI~YfC SPECIFICATIONS). These feature inventories as well as a large amount of already existing, partly fairly traditional work on semantic feature systems of linguistics and philosophy (CHAFE, FRIEDRICH, PAIMER, VENDLER) , of information sciences (DAI~BEI~), and work in the field of cog= nitive linguistics and artificial intelligence (G.Ao MI~, G.A. MI~ & P.N. JOHNSON-IAIRD, G. LAKOFF, R. LANGACKER, B. COHEN, W.R. GARNER, ATINEAVE, FRED~IKSEN, MINSKY, (~N/AK, WINOGRAD, ANDERSON & BOWER, WOODS) and last but not least scme recent issues on word semantics (T. BAIIMER, W. BRENNENS~IHL, J.BALLWEG, H. FROSCH) have been taken into account in order to meet the requiremerits of a manageable syst~n of semantic features. This system is intended both to be based on a sensible theory of semantics and to satisfy the special requirements of machine translation in general and of our t~t type in particular. Moreover, it should be flexible enough to be enlarged and supplemented or changed, whenever this proves necessary on empirical evidence = this last requir~rent being made possible by the accc~plishment of the first. In tmying to meet the~ requirea~nts the semantic feature inventories ~t hand have been enlarged, changed, and adapted to our specific purposes m~d have been merged into one system of semantic features. 2. Comment on the theoretical asslmi0tions made in different machine translation systems with respect to the semantic representation with respect to the semantic representation which in EUROTRA will be implemented on the interface structure (IS) level of the source and target language it is our first and foremost aim to arrive at a coherent system of semantic features. In order not to start from nothing the above mentioned feature inventories have l~en consulted. The feature inventories developed fer these machine translation systems have different shortcomings which will be briefly comm~%ted on in the following. since a sufficient def:hnition of how to interpret the features is given J/% none of the proposals of the above mentioned mac/line translation systems, we will not ccm~ent here on the ~eatures the~nselves included in the propo~ils. A brief ccsment, however, is necessary on 'the general approach, which seems to imply theoretical assumptions (not explicitly mentioned, since neither a theoretical nor a practical usage-based ~101anation is given) about the organisation and processing of semantic units, for which there is no e~pirical evidence: neither natural language processing by human beings nor efficiency in automatic processing of natural language gives support to i:~ese implied assumptions. It must be mentioned, however, that this can by no means be considered to be an objective comment, since for an outsider, it is ~/~oossible to urgerstand the systematic aDtivation of these feature inventories for at least one of the following reasons: The semantic features are not defined or at least not sufficiently defJmed in order to n~ke clear their conceptual structure and thus to make clear how they are meant to be used. This is especially true for the EUROTRA proposal, in ~lich semantic features are not defined at all. rfhis is, however, only a proposal, which has not been applied yet, but is being tested at the. mcment. But also the SYSTRAN semantic features, as well as those of JAPAN, which have been worked out rather sophistieatedly, are not cc~_nted on. The semantic features of METAL are defined, their definition, however, remains rather vague. Even when taking into consideration the ~les which are added, the reader does not arrive at a satisfactory understa~. The dependencies holding between features are. not explained. This is especially true for SYSTRAN, which only gives a list of features z~ferring to arguments. A hierarchical system consisting of two levels of semantic features is defined by METAL, which is far frc~ sufficient. JAPAN J s worked out in a more sophisticated way with respect to this problem. Both in METAL and in JAPAN, however, relations between the dominating features are not defined. The ~ proposal gives an enumeration on the second and lowest le~ vel of the feature tree, which is just a conglon~ration of semantic information, which should be described at different levels~ in order to achieve the overall aim of linguistically consistent semantic description. 3. A proposal for a EUROTRA semantic feature rule system 3. i. Necessity of a semantic feature rule system Let us now put forward our conception of the ~o system~ ef ~tic features with respect to its fom~alization. We have two gr~, one describing "SITUATION" features, the other one describing "~TITY" features. Neither of the two systems is strictly hierarchically organized. The hierarchical principle, however, which always defines a refinement of the doafinat~/~g feattzt~, prevails. Particularly the most general semantic features, such as the "ENTITY" features "CONC~ETE/"ABSTRACT", "CC~JNTABLE"/'9~ASS", and "NAIURAL"/"ARTIFICIAL", and the "SITUATION" features "OONCREI~"/"ABSTRAC~", "STATIVE"/"DYNAMIC", and "H/NCIUAL"/ "DURATIVE"/"ITERATIVE", respectively, form pairs or triplets of semantic features. One feature of each of these alternations obligatorily occurs, and the descendents, which specify them, form disjunct sets.2 3.2. The basic formalis~a Let us now comment informally on our present conception of how the semantic features which we consider necessary so far are related to each ot2ner. We use three operations holding between semantic features in our ¢/rammar: l) Hierarc/Iy is the overall relation defining the derivation of the features. 2) Alternation relates a set of features, only one of which applies. 3) ~ relates semantic features obligatorily occurrJ/~g together. This type of ralationship is of course in the minority. The basic idea is to describe these relations by a context-free ~ule system, where the rules can for example be of the following, folul~: (3. i) X = (A/B)* (C/D) The hier~ly here is repz~sented by t/~e sign "=", the alternation by the sign "/", and the disjunction by the sign "*". The interpretation of the rule is the following: The feature on the left handside of the rtlle dc~ minates the features appearing o11 the right handside° A, B, C, and D establish a refinement of X. Mere precisely, in this example X is specified by a pair of features, the first ~nent of which can be either A or B and the .~econd is either C or Do ~he subordinate feattlre~ on the right llandside of the tulle can get supe~rdinate feature~ themselve~ o11 the next level lower dowel in the hie/-archy. ~e terr~inal featuz~s~ t/iat is those features which are not defined for accepting a/~y ~ulbordinat~ features, are rep ~resented by the rules