| { |
| "paper_id": "P81-1017", |
| "header": { |
| "generated_with": "S2ORC 1.0.0", |
| "date_generated": "2023-01-19T09:15:16.751834Z" |
| }, |
| "title": "What's Necessary to Hide?: Modeling Action Verbs", |
| "authors": [ |
| { |
| "first": "James", |
| "middle": [ |
| "F" |
| ], |
| "last": "Alien", |
| "suffix": "", |
| "affiliation": {}, |
| "email": "" |
| } |
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| "year": "", |
| "venue": null, |
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| "abstract": "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. 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\u2022 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. 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. 1979]. As interest in these methods grows (e.g., see [Grosz, 1979; Brachman, 1979]). the inadequacy of existing action models becomes increasingly obvious.", |
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| "paper_id": "P81-1017", |
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| "abstract": [ |
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| "text": "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. 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\u2022 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. 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. 1979]. As interest in these methods grows (e.g., see [Grosz, 1979; Brachman, 1979]). the inadequacy of existing action models becomes increasingly obvious.", |
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| "section": "Abstract", |
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| "text": "The formalism for actions used in most natural language understanding systems is based on case grammar. Each action is represented by a set of assertions about the \u2022 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.", |
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| "text": "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, \"Walking to the store while juggling three bails\" seems to be composed of the actions of \"walking to the store and \"juggling three bails.\"", |
| "cite_spans": [ |
| { |
| "start": 91, |
| "end": 111, |
| "text": "[I:ikes and Nilsson,", |
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| { |
| "start": 112, |
| "end": 116, |
| "text": "197]", |
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| "start": 119, |
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| "text": "Sacerdoti, 1975]", |
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| "text": "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.", |
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| "text": "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", |
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| "text": "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.", |
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| "text": "There are few physical activities that are a necessary part of performing this action, Depending on the context, vastly different patterns or\" 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 \"turning on the light\" even if we had never seen or thought about that pattern previously. 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. 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\u2022 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] \u2022 Other areas axe currently under investigation.", |
| "cite_spans": [ |
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| "start": 954, |
| "end": 971, |
| "text": "Jackendoff [1976]", |
| "ref_id": "BIBREF13" |
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| "start": 1911, |
| "end": 1924, |
| "text": "Allen [198.1]", |
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| "text": "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.", |
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| "section": "7'7 /\"", |
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| "text": "Before we can characterize events and actions, we need to specify a temporal logic. The logic described here is based on temporal intervals. Events that appear to refer to a point in time (i.e., finishing a race) are considered to be implicitly referring to another event's beginning or ending. Thus the only time points we will see will be the endpoints of intervals.", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "A Temporal l,ogie", |
| "sec_num": "2." |
| }, |
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| "text": "The logic is a typed first order predicate calculus, in which the terms fall into the following three broad categories:", |
| "cite_spans": [], |
| "ref_spans": [], |
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| "section": "A Temporal l,ogie", |
| "sec_num": "2." |
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| "text": "-terms of type TIME-INTERVAL denodng time intervals;", |
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| "section": "A Temporal l,ogie", |
| "sec_num": "2." |
| }, |
| { |
| "text": "terms of type PROPERTY, denoting descriptions that can hold or not hold during a particular time; and terms corresponding to objects in the domain.", |
| "cite_spans": [], |
| "ref_spans": [], |
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| "section": "A Temporal l,ogie", |
| "sec_num": "2." |
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| "text": "There are a small number of predicates. One of the most important is HOLDS, which asserts that a property holds (i.e., is true) during a time interval..Thus", |
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| "section": "A Temporal l,ogie", |
| "sec_num": "2." |
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| "text": "is true only if property p holds during t. As a subsequent axiom will state, this is intended to mean that p holds at every subinterval oft as well.", |
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| "section": "HOLDS(#,O", |
| "sec_num": null |
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| "text": "There is no need to investigate the behavior of MEETS(tl, t2)--interval tl is before interval 12, but there is no interval between them, i.e., tl ends where t2. starts. Given these predicates, there is a set of axioms defining their interrelations. For example, there are axioms dealing with the transitivity of the temporal relationships. Also, there is the axiom mentioned previously when the HOI,I)S predicate wa~ introduced: namely HOI,DS(p.tl) This gives us enough tools to define the notion of action in the next section.", |
| "cite_spans": [ |
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| "start": 436, |
| "end": 448, |
| "text": "HOI,DS(p.tl)", |
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| "section": "HOLDS(#,O", |
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| "text": "(A.]) IfOLDS(p.t) & DURING(tl.t) --)", |
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| "section": "HOLDS(#,O", |
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| "text": "In order to define the role that events and actions play in the logic, the logical form of sentences asserting that an event has occurred must be discussed. Once even~ have been defined, actions will be defined in terms of them. One suggestion for the logical form is to define for each c[,,~ of events a property such that the property HOI.I)S only if the event occurred. This can be discarded immediately as axiom (A.]) is inappropriate for events. If an event occurred over some time interval \"['. it does not mean that the event also occurred over all subintervals of T. So we introduce a new type of object in the logic, namely events, and a new predicate OCCUlt. l),y representing events as objects in the logic, we have avoided the difficulties described in Davidson [1967] .", |
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| "text": "Davidson [1967]", |
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| "section": "Events and Actions", |
| "sec_num": "3." |
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| "text": "Simply giving the logical form of an event is only a small part of the analysis. We must also define for each event the set of conditions that constitute its occurrence. As mentioned in the introduction, there seems to be no restriction on what kind of conditions can he used to define an event except that they must partially describe the world over some time interval.", |
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| "section": "Events and Actions", |
| "sec_num": "3." |
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| "text": "For example, the event \"the ball moving from x to y\" could be modeled by a predicate MOVE with four arguments: the object, the source, the goal location, and the move event itself. Thus, MOVI' (IlalL x. y. m) asserts that m is an event consisting of the ball moving from x to y. We assert that this event occurred over time t by adding the assertion", |
| "cite_spans": [ |
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| "start": 193, |
| "end": 208, |
| "text": "(IlalL x. y. m)", |
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| "section": "Events and Actions", |
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| "text": "With these details out of the way. we can now define necessary and sufficient conditions for the event's occurrence. For this simple class of move events, we need an axiom such as: A simple class of events consists of those that occur only if some property remains constant over a particular interval (c\u00a3 Jackendoffs STAY verbs). For example, we may assert in l'nglish \"The ball was in the room during T.'\" \"The ball remained in the room during T.\"", |
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| "section": "OCCUR(,~ t).", |
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| "text": "(forall object,", |
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| "text": "While these appear to be logically equivalent, they may have very different consequences in a conversation. This formalism supports this difference. The former sentence asserts a proposition, and hence is of the form", |
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| "section": "OCCUR(,~ t).", |
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| "text": "while the latter sentence describes an event, and hence is of the form", |
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| "section": "H O L D S(in( BalI, R oom), T)", |
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| "text": "We may capture the logical equivalence of the two with the axiom: O'orall b.r,e,O REMAIN-IN(b,r,e) ", |
| "cite_spans": [ |
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| "start": 82, |
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| "text": "REMAIN-IN(b,r,e)", |
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| "section": "REMAIN-IN(Bail, Room, e) & OCCURS(e T).", |
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| "text": "The problem remains as to how the differences between these logically equivalent formulas arise in context. One possible difference is that the second may lead the reader to believe that it easily might not have been the case.", |
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| "section": "& OCCUR(nO (=) HOL1)S(in(b.r),O,", |
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| "text": "Actions are events that involve an agent in one of two ways. The agent may cause the event or may allow the event (cf. [Jackendoff, 1976] ). Corresponding to these two types of agency, there are two predicates, ACAUSE and ALLOW, that take an agent, an event, and an action as arguments. Thus the assertion corresponding to \"John moved 13 from S to G\" is MO VE (B, G,S, el) The remainder of this paper applies the above formalism to the analysis of the action of hiding a book from someone. Along the way, we shall need to introduce some new representational tools for the notions of belief, intention, and causality,", |
| "cite_spans": [ |
| { |
| "start": 119, |
| "end": 137, |
| "text": "[Jackendoff, 1976]", |
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| "text": "(B, G,S, el)", |
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| "section": "& OCCUR(nO (=) HOL1)S(in(b.r),O,", |
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| "text": "The definition of hiding a book should be independent of any method by which the action was performed, for, depending on the context, the actor could hide a book in many different ways. For instance, the actor could put the book behind a desk, -stand between the book and the other agent while they are in the same room, or call a friend Y and get her or him to do one of the above.", |
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| "text": "Furthermore, the actor might hide ).he book by simply not doing something s/he intended to do. I:or example, assume Sam is planning to go to lunch with Carole after picking Carole up at Carole's office, if, on the way out of Sam's office, Sam decides not to take his coat because he doesn't want Carole to see it, then Sam has hidden the coat from Carole. Of course, it is crucial here that Sam believed that he normally would have taken the coat. Sam couldn't have hidden his coat by forgetting to bring it.", |
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| "text": "This example brings up a few key points that may not be noticed from the first three examples. First' Sam must have intended to hide the coat. Without this intention (i.e., in the forgetting case), no such action occurs. Second, Sam must have believed that it was likely that Carole would see the coat in the future course of events. Finally, Sam must have acted in such a way that he then believed that Carole would not see the coat in the future course of events. Of course, in this case, the action Sam performed was \"not bringing the coat,\" which would normally not be considered an action unless it was intentionally not done. I claim that these three conditions provide a reasonably accurate definition of what it means to hide something. They certainly cover the four examples presented above. As stated previously, however, the definition is rather unsatisfactory, as many extremely difficult concepts, such as belief and intention, were thrown about casually.", |
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| "section": "& OCCUR(nO (=) HOL1)S(in(b.r),O,", |
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| "text": "There is much recent work on models of belief (e.g., [Cohen, 1978; Moore, 1979; Perils, 1981 \" Haas, 1981 ). l have little to add to these efforts, so the reader may assume his or her favorite model. I will assume that belief is a modal operator and is described by a set of axioms along the [iu~ of Hintikka [I962] . The one important thing to notice, though, is that there are two relevant time indices to each belief; namely, the time over which the belief is held, and the time over which the proposition that is believed holds. For example. I might believe ~oda.v that it rained last weekend. This point wiil be crucial in modeling the action of hiding. To introduce some notation, let \"A believes (during To) that p holds (during Tp)\" be expressed as", |
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| "start": 53, |
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| "text": "Moore, 1979;", |
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| "text": "\" Haas, 1981", |
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| "text": "The notion of intention is much less understood than the notion of belief. However, let us approximate the statement \"A intends (during Ti) that action a happen (during Ta)\" by and \"A believes (during Ti)that a happen (during Ta)\" \"A wants (during Ti) that a happen (during Ta)\" This is obviously not a philosophically adequate definiuon (e.g., see [Searle, 1980] ), but seems sufficient for our present purposes. The notion of wanting indicates that the actor finds the action desirable given the alternatives. This notion appears impossible to axiomatize as wants do not appear to be rational (e.g. Hare []97]]). However, by adding the belief that the action will occur into the notion of intention, we ensure that intentions must be at least as consistent as beliefs.", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "Actions may be performed intentionally or unintentionally. For example, consider the action of breaking a window. Inferring intentionality from observed action is a crucial ability needed in order to communicate and cooperate with other agents. While it is difficult to express a logical connection between action and intention, one can identify pragmatic or plausible inferences that can be used in a computational model (see [Allen, 1979] ).", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "With these tools, we can attempt a more precise definition of hiding. The time intervals that will be required are:", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "Th--the time of the hiding event;", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "Ts--the time that Y is expected to see the book;", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "Tbl--the time when X believes Y will see the book during \"l's, which must be BEFORE \"l'h;", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "Tb3--the time when X believes Y will not see the book during Ts, which must be BEI\"ORE or DURING Th and AI\"I'I'~R Tbl.", |
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| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
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| "text": "We will now define the predicate H I D I. '(agent, observer, object, a~t) which asserts that act is an action of hiding. Since it describes an action, we have the simple axiom capturing agency: (forall agent, observer, obJect, act H I D l:'(agent, observer, object, act) =) (Exists e ACAUSE(agent, e, act)))", |
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| "text": "l.et us also introduce an event predicate S E l:'(agent, object, e) which asserts that e is an event consisting of agent seeing the object.", |
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| "text": "Now we can define HIDE as follows: (forall ag, obs, o.a. 77z, HIDl'.'(ag.obs, o, a) (obs, o,e) and the intervals Th, Ts, Tb], Tb3 are related as discussed above. Condition (4) defines e as a seeing event, and might also need to be within ag's beliefs.", |
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| "start": 35, |
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| "text": "This definition is lacking part of our analysis; namely that there is no mention that the agent's beliefs changed because of something s/he did. We can assert that the agent believes (between Tbl and Tb3) he or she will do an action (between Tbl and Th) as follows: (existx\" al, el, Tb2 5) ACAUSlf(a&el, aD 6) H O LDS(believes(ag, OCC UR(al, Tal) ), Tb2) where 7\"b1 ( Tb2 ( Tb3 and Tbl (", |
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| { |
| "start": 266, |
| "end": 278, |
| "text": "(existx\" al,", |
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| "start": 279, |
| "end": 282, |
| "text": "el,", |
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| { |
| "start": 283, |
| "end": 303, |
| "text": "Tb2 5) ACAUSlf(a&el,", |
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| }, |
| { |
| "start": 304, |
| "end": 330, |
| "text": "aD 6) H O LDS(believes(ag,", |
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| }, |
| { |
| "start": 331, |
| "end": 341, |
| "text": "OCC UR(al,", |
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| "start": 342, |
| "end": 346, |
| "text": "Tal)", |
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| "eq_spans": [], |
| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
| "sec_num": null |
| }, |
| { |
| "text": "But this has not caused the change in (3) are true, asserting Tal ( Tit captured the notion that belief (6) belief from (2) to (3). Since (6) and a logical implication from (6) to (3) would have no force. It is essential that the belief (6) be a key-element in the reasoning that leads to belief (3).", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
| "sec_num": null |
| }, |
| { |
| "text": "To capture this we must introduce a notion of causality. This notion differs from ACAUSE in many ways (e.g. see [Taylor, 1966] ), but for us the major difference is that, unlike ACAUSE, it suggests no relation to intentionality. While ACAUSE relates an agent to an event, CAUSE relates events to events. The events in question here would be coming to the belief (6), which CAUSES coming to the belief (3).", |
| "cite_spans": [ |
| { |
| "start": 112, |
| "end": 126, |
| "text": "[Taylor, 1966]", |
| "ref_id": "BIBREF20" |
| } |
| ], |
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| "eq_spans": [], |
| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
| "sec_num": null |
| }, |
| { |
| "text": "One can see that much of what it means to hide is captured by the above. In particular, the following can be extracted directly from the definition: -if you hide something, you intended to hide it, and thus can be held responsible for the action's consequences;", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
| "sec_num": null |
| }, |
| { |
| "text": "one cannot hide something if it were not possible that it could be seen, or if it were certain that it would be seen anyway; -one cannot hide something simply by changing one's mind about whether it will be seen.", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
| "sec_num": null |
| }, |
| { |
| "text": "In addition, there ate many other possibilities related to the temporal order of events. For instance, you can't hide something by performing an action after ,,he hiding is supposed to be done.", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "H O LDS(believes(A. holde(p. Tp)), Tb).", |
| "sec_num": null |
| }, |
| { |
| "text": "I have introduced a representation for events and actions that is based on an interval-based temporal logic. This model is sufficiently powerful to describe events and actions that involve change, as well as those that involve maintaining a state. In addition, the model readily allows the composition and modification of events and actions.", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "Conclusion", |
| "sec_num": null |
| }, |
| { |
| "text": "In order to demonstrate the power of the model, the action of hiding was examined in detail. This forced the introduction of the notions of belief, intention, and causality. While this paper does not suggest any breakthroughs in representing these three concepts, it does suggest how they should interact with the notions of time, event, and action.", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "Conclusion", |
| "sec_num": null |
| }, |
| { |
| "text": "At present, this action model is being extended so that reasoning about performing actions can be modeled. This work is along the lines described in [Goldman, 1970] .", |
| "cite_spans": [ |
| { |
| "start": 149, |
| "end": 164, |
| "text": "[Goldman, 1970]", |
| "ref_id": null |
| } |
| ], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "Conclusion", |
| "sec_num": null |
| } |
| ], |
| "back_matter": [ |
| { |
| "text": "The author wishes to thank Jerry Feldman, Alan l:risch, Margery I.ucas, and Dan I,',ussell for many enlightening comments on previous versions of this paper. ", |
| "cite_spans": [], |
| "ref_spans": [], |
| "eq_spans": [], |
| "section": "Acknowledgements", |
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| "FIGREF0": { |
| "text": "fully here. but in Allen [forthcomingJ various functional forms are defined that can be used within the scope of a HOLDS predicate that correspond to logical connectives and quantifiers outside the scope of the HOLDS predicate. There is a basic set of mutually exclusive relations that can hold between temporal intervals. -Each of these is represented by a predicate in the logic..The most important are: DURING(tl, t2)--time interval tl is fully contained within 12, although they may coincide on their endpoints. BEFORE(tl,t2)--time interval t] is before interval 12, and they do not overlap in any way: OVERLAP(tl, t2)--interval tl starts before t2, and they overlap;", |
| "uris": null, |
| "num": null, |
| "type_str": "figure" |
| } |
| } |
| } |
| } |
