Passives

The English passive construction has played a central role in the to-ings and fro-ings of grammatical theory over the last 30 years, from the earliest days of transformational grammar, to more recent, surface oriented theories of syntax. The casual reader of the linguistic literature might therefore suppose that the computational linguist looking for an off the shelf analysis of passives would be able to choose from among several competing analyses, each of which accommodated the facts, but perhaps derived them from (or from them} different theoretical principles. Unfortunately, this is not the case. as we shall see. All of the analyses that [ am familiar with are incomplete, or inaccurate in some respects, or simply unprogrammable in any straightforward form. The present paper is an at tempt to remedy this situation, and to provide such an off the shelf analysis of the syntax and semantics of passives. The analysis of this central construction will be couched within a simple and computationally tractable syntactic and semantic formalism, and should translate easily to most currently popular formalisms. It will be quite eclectic, freely borrowing from several different grammatical theories. T w o u n s a t i s f a c t o r y ana lyses The original starting point for the analysis here was that presented in Gazdar et al. 1985 (also found unsatisfactory by Kilbury t986). In the GPSG framework, passive VP rules are derived by a metarule from active VPs: 1. VP NP. W ~ VPpas -V~. (PPby) The interpretation of this metarule is as follows: for every rule expanding VP which introduces an NP daughter. there is also to be a rule which has the VP marked as passive, does not contain that NP daughter, and may contain a PP headed 'by'. Feature principles ensure that the verb heading the VP will have passive morphology in this latter c a s e . There are several problems with this account. An engineering problem concerns the interpretation of GPSGs for computational purposes. One more or less workable construal regards the metagrammar as a set of instructions for producing a 'compiled" object grammar consisting of context free rules augmented with some feature matching mechanism. However, this treatment produces large numbers of such rules. When 'slashed' versions of VP rules are also compiled out the multipllcative effect can lead to many hundreds of VP rules in a linguistic description. While not fatal, this is still a problem for constructing efficient parsers. There are also several descriptive problems. As Kilbury points out. the metarule as it stands would apply to VPs which require a sentential subject, like "bother'. on one of its subcategorisations. Thus we will be able to generate junk like: 2. That Kim left was bothered [by Sandy). Similarly. for VPs introducing complements of verbs like "elect' we will get two outputs from the metarule, only one of which is a sensible one: a. v P v(211 .~'m s P ( 'etecc etc are VI211 ) 4. a We elected Kim president b Kim was elected president c *President was elected Kim The metarule wilL, however, fail to apply in the case of VPs introducing a sentential object, since there is no NP daughter, failing to generate perfectly good examples like 6b: 5. V P V . . S" 6. a They vehemently denied that there had been a plutonium leak b That there had been a plutonium leak was vehemently denied. Most of these problems are fairly easily fixable: for examples like 2. it is a reasonable response to say that they are syntactically ok. but sortally deviant: the obvious fix for things like 6 is to regard sentential complements of this