Using Situation Descriptions and Russellian Attitudes for Representing Beliefs and Wants

A representat ion scheme fo r a rb i t r a r y be l ie fs and wants of an agent in respect to a s i t u a t i o n , as well as to a r b i t r a r y be l ie fs and wants of other agents, is presented. The representat ion makes use of elementary s i t ua t i on descr ipt ions (which are formulated in KL-ONE and de l imi ta ted by p a r t i t i o n s ) , and acceptance a t t i tudes in respect to these descr ip t ions , or to a t t i t udes thereabout. The scheme forms the representa t i ona l base of VIE-DPM, the user modell ing component of the German-language dialogue system VIE-LANG. 1 . In t roduc t ion Three d e t a i l l e d proposals have been made up to now fo r representing a system's assumptions about the be l i e f s and wants of another agent: In the "syntact ic approach" [ 9 ] , be l i e f of an agent is equated with d e r i v a b i l i t y in a f i r s t o r d e r object-language theory of the agent. In the "semantic approach" [ 1 0 , 1 ] , knowledge and wants are equated with a c c e s s i b i l i t y re la t i ons between possible worlds. F i n a l l y , in the " p a r t i t i o n approach" [ 4 ] , be l i e f s and wants of an agent are equated wi th the presence of representat ion st ructures in spec i f i c nested be l i e f and want spaces reserved fo r the respective agent. In the f i e l d of epistemic l og i c , be l i e f s and wants of an agent have o r i g i n a l l y been regarded as "propos i t iona l a t t i t u d e s " , i . e . a t t i tudes of the agent in respect to cer ta in proposit ions about the world [11 ,12 ] . In t h i s paper, a new representat ion scheme fo r be l i e f s and wants w i l l be presented which is based on a s im i la r idea, and which also integrates aspects of the p a r t i t i o n approach. The proposed scheme forms the representat ional base fo r the user modell ing component VIE-DPM [ 6 , 8 ] , which is part of the German-language dialogue system VIE-LANG [ 1 5 ] . Our representat ion fo r be l i e f s and wants is thus f u l l y in tegrated into a natural-language environment. 2. S i tua t ions and s i t ua t i on descr ip t ions In our representat ion scheme, basic be l ie fs and wants of an agent ( i . e . be l i e f s and wants which do not concern other agents' be l i e f s and wants) are regarded as a t t i t udes which the agent holds in respect to s i t ua t i on descr ip t ions , e i ther of the current The research reported here has been supported by the Austr ian Min is t ry of Science (contract No. 607-001). I am indebted to Stuart S. Shapiro for h is comments on an e a r l i e r version of t h i s paper. s i t u a t i o n , or in the case of wants, of a s i t u a t i o n which is aspired by the agent. A s i t u a t i o n , as we define i t , is a set of ind iv idua ls and re l a t i ons between ind i v idua ls . The former wi l l be denoted by ' i ' , ' i 1 ' , e t c . , the l a t t e r b y ' r ' , ' r 1 ' , e tc . I f there is a re l a t i on r ( i 1 , i2) in the s i t u a t i o n , the pair ( r , i2) is ca l led an a t t r i b u t e of i 1 . For descr ibing s i t ua t i ons , a descr ip t ion language is needed. In our representat ion, the i n d i v i d u a l ized level of the KL-ONE formalism [ 2 ] is used f o r t h i s purpose. Minor re i n te rp re ta t i ons have been necessary in order to adjust t h i s scheme to the spec i f i c needs of be l ie f and want representat ion. We assume the reader to be roughly f am i l i a r wi th the KL-ONE philosophy, so that we can r e s t r i c t ourselves to exp l i c a t i n g the ro le of KL-ONE in our own representat i on scheme. Fig.1 shows a very s imp l i f i ed example of our r e presentat ion, expressed in the usual network notat i o n . Layer A forms i t s conceptual l e v e l , layer B i t s ind iv idua l ized l e v e l . For our purposes, i t su f f i ces to regard the former as consis t ing of general concepts, and general a t t r i b u t e descr ipt ions associated wi th general concepts. General concepts can be r e garded as one-place predicates which apply to i n d i v idua ls . Fig.1 exempl i f ies, that more and less spec i f i c concepts can be ordered in a superconcept h i e r archy. A general a t t r i b u t e descr ip t ion consists of a ro le ( fo r our purposes, a one-place predicate applying to re la t i ons ) and a value r e s t r i c t i o n concept f o r ro le f i l l e r s . The l a t t e r can i t s e l f posses associated a t t r i b u t e descr ip t ions , e tc . More advanced aspects of KL-ONE's general level w i l l not be considered here, since they are not as important fo r be l i e f represent a t i o n . Structures of the ind iv idua l ized level are created by assigning ind iv idua l ized concepts and ind iv idua l i zed a t t r i b u t e descr ipt ions to t he i r general counterparts. Through th i s process of i n d i v i d u a l i z a t i o n , elementary s i t ua t i on descr ipt ions are created: The i nd i v i dua l i za t i on 57 the general concept c in to the ind iv idua l i zed concept ic expresses that an ind iv idua l i ex is ts in the s i t ua t i on to which c appl ies. The i nd i v i dua l i za t i on of the general r o l e gr in to the ind iv idua l ized ro le i r expresses that the r e l a t i o n r ex is ts in the s i t u a t i o n , to which gr appl ies . In both cases, ic and ir are said to designate t h i s ind iv idua l or r e l a t i o n , respec t i ve ly . In F i g . 1 , the general layer has been i n d i v i d u a l ized three t imes. Each substructure expresses the fo l l ow ing elementary descr ip t ions : ind iv idua ls 11-16 ex is t in the s i t u a t i o n , to which the predicates 'OBJTRANS', 'JOHN', 'MARY' ( 'SUE') , 'BOOK', 'USER' and 'SYSTEM' apply, respect ive ly (the l a t t e r has been l e f t out in the central and r ight-hand subst ruc ture) .