The Imperfective Paradox and Trajectory-of-Motion Events

THE IMPERFECTIVE PARADOX AND TRAJECTORY-OF-MOTION EVENTS * Michael White Department of Computer and Information Science University of Pennsylvania Philadelphia, PA, USA mwhit e©l inc. c is. upenn, edu Abstract In the first part of the paper, I present a new treatment of THE IMPERFI~CTIVE PARADOX (Dowty 1979) for the restricted case of trajectory- of-motion events. This treatment extends and re- fines those of Moens and Steedman (1988) and Jackendoff (1991). In the second part, I describe an implemented algorithm based on this treatment which determines whether a specified sequence of such events is or is not possible under certain sit- uationally supplied constraints and restrictive as- sumptions. Introduction Bach (1986:12) summarizes THE IMPERFECTIVE PARADOX (Dowty 1979) as follows: "...how can we characterize the meaning of a progressive sen- tence like (la) [17] on the basis of the meaning of a simple sentence like (lb) [18] when (la) can be true of a history without (lb) ever being true?" (la) John was crossing the street. (lb) John crossed the street. Citing parallels in the nominal domain, Bach goes on to point out that this puzzle is seemingly much more general, insofar as it appears whenever any sort of partitive is employed. In support of this view, we may observe that the start v-ing con- struction exhibits the same behavior: (2a) John started jogging to the museum. (2b) John jogged to the museum. Here we see that (2a) does not entail (2b) -- while (2b) asserts the occurrence of an entire event of John jogging to the museum, (2a) only asserts the *The author gratefully acknowledges the helpful comments of Mark Steedman, Jeff Siskind, Christy Doran, Matthew Stone, and the anonymous refer- ees, as well as the support of DARPA N00014-90-J- 1863, AI~O DAAL03-89-C-0031, NSF IRI 90-16592, Ben Franklin 91S.3078C-1. occurrence of the beginning of such an event, leav- ing open the existential status of its completion. Capitalizing on Bach's insight, I present in the first part of the paper a new treatment of the imperfective paradox which relies on the pos- sibility of having actual events standing in the part-of relation to hypothetical super-events. This treatment extends and refines those of Moens and Steedman (1988) and Jackendoff (1991), at least for the restricted case of trajectory-of-motion events. 1 In particular, the present treatment cor- rectly accounts not only for what (2a) fails to en- tail -- namely, that John eventually reaches the museum -- but also for what (2a) does in fact en- tail -- namely, that John follows (by jogging) at least an initial part of a path that leads to the museum. In the second part of the paper, I briefly describe an implemented algorithm based on this theoretical treatment which determines whether a specified sequence of trajectory-of-motion is or is not possible under certain situationally supplied constraints and restrictive assumptions. Theory The present treatment builds upon the ap- proach to aspectual composition developed in White (1993), a brief sketch of which follows. White (1993) argues that substances, processes …

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