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<Paper uid="P81-1017">
  <Title>What's Necessary to Hide?: Modeling Action Verbs</Title>
  <Section position="1" start_page="0" end_page="0" type="abstr">
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Ahstract
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    <Paragraph position="0"> This paper considers what types of knowledge one must possess in order to reason about actions. Rather than concentrating on how actions are performed, as is done in the problem-solving literature, it examines the set of conditions under which an action can be said to have occurred. In other words, if one is told that action A occurred, what can be inferred about the state of the world? In particular, if the representation can define such conditions, it must have good models of time, belief, and intention. This paper discusses these issues and suggests a formalism in which general actions and events can be defined. Throughout, the action of hiding a book from someone is used as a motivating example.</Paragraph>
    <Paragraph position="1"> I. Introductio, This paper suggests a formulation of events and actions that seems powerful enough to define a wide range of event and action verbs in English. This problem is interesting for two reasons* The first is that such a model is necessary to express the meaning of many sentences. The second is to analyze the language production and comprehension processes themselves as purposeful action.</Paragraph>
    <Paragraph position="2"> This was suggested some time ago by Bruce \[1975\] and Schmidt \[1975\]. Detailed proposals have been implemented recently for some aspects of language production \[Cohen, 1978\] and comprehension \[Alien.</Paragraph>
    <Paragraph position="3"> 1979\]. As interest in these methods grows (e.g., see \[Grosz, 1979; Brachman, 1979\]). the inadequacy of existing action models becomes increasingly obvious.</Paragraph>
    <Paragraph position="4"> The formalism for actions used in most natural language understanding systems is based on case grammar.</Paragraph>
    <Paragraph position="5"> Each action is represented by a set of assertions about the * semantic roles the noun phrases play with respect to the verb. Such a tbrmalism is a start, but does not explain how to represent what an action actually signifies. If one is told that a certain action occurred, what does one know about how the world changed (or didn't change!). This paper attempts to answer this question by oudining a temporal logic in which the occurrence of actions can be tied to descriptions of the world over time.</Paragraph>
    <Paragraph position="6"> One possibility for such a mechanism is found in the work on problem-solving systems (e.g. \[I:ikes and Nilsson, 197\]; Sacerdoti, 1975\]), which suggests one common formulation of action. An acuon is a function from one world state to a succeeding world state and is described by a set of prerequisites and effects, or by decomposition into more primitive actions. While this model is extremely useful for modeling physical actions by a single actor, it does not cover a large class of actions describable in I-ngiish. \[:or instance, many actions seemingly describe nml-activity (e.g. standing still), or acting in some nonspecified manner to preserve a state (e.g. preventing your televismn set from being stolen). Furthermore, many action descriptions appear to be a composition of simpler actions that are simultaneously executed. For instance, &amp;quot;Walking to the store while juggling three bails&amp;quot; seems to be composed of the actions of &amp;quot;walking to the store and &amp;quot;juggling three bails.&amp;quot; It is not clear how such an action could be defined from the two simpler actions if we view actions as functions from one state to another.</Paragraph>
    <Paragraph position="7"> The approach suggested here models events simply as partial descriptions of the world over some Lime interval. Actions are then defined as a subclass of events that involve agents. Thus, it is simple to combine two actions into a new action, The new description simply consists of the two simpler descriptions hglding over the same interval The notions of prerequisite, result, and methods of performing actions will not arise in this study. While they are iraportant for reasoning about how to attain goals, they don't play an explicit role in defining when an action can be said to have occurred. To make this point clear, consider the simple action of turning on a light.</Paragraph>
    <Paragraph position="8"> There are few physical activities that are a necessary part of performing this action, Depending on the context, vastly different patterns or&amp;quot; behavior can be classified as the same action, l;or example, turning on a light usually involves Hipping a light switch, but in some circumstances it may involve tightening the light bulb (in the basement). or hitting the wail (m an old house). Although we have knowledge about how the action can be pertbrmed, this does nol define what the action is. The key defining characteristic of turning on the light seems to be that the agent is performing some activity which will cause the light, which is off when the action starts, to become on when the action ends. The importance of this observation is that we could recognize an observed pattern of activity as &amp;quot;turning on the light&amp;quot; even if we had never seen or thought about that pattern previously.</Paragraph>
    <Paragraph position="9"> The model described here is in many ways similar to that of Jackendoff \[1976\]. He provides a classification of event verbs that includes verbs of change (GO verbs) and verbs that assert a state remaining constant over an interval of time (STAY verbs), and defines a representation of action verbs of both typesby introducing the notion of agentive causality and permission. However, Jackendoff does not consider in detail how specific actions might be precisely defined with respect to a world model.</Paragraph>
    <Paragraph position="10"> The next two sections of this paper will introduce the temporal logic and then define the framework for defining events and actions. To be as precise as possible, I have remained within the notation of the first order predicate calculus* Once the various concepts are precisely defined, the next necessary step in this work is to define a computaUonally feasible representation and inference process, Some of this work has already been done. For example, a computational model of the temporal logic can be found in Allen \[198.1\]* Other areas axe currently under investigation.</Paragraph>
    <Paragraph position="11">  /&amp;quot; The final section demonstrates the generality of the approach by analyzing the action of hiding a book from someone. In this study, various other important conceptual entities such as belief, intention, and causality are briefly discussed. Finally, a definition of.what it means to hide something is presented using these tools.</Paragraph>
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