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<Paper uid="P81-1017">
  <Title>What's Necessary to Hide?: Modeling Action Verbs</Title>
  <Section position="3" start_page="0" end_page="78" type="intro">
    <SectionTitle>
3. Events and Actions
</SectionTitle>
    <Paragraph position="0"> 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.</Paragraph>
    <Paragraph position="1"> 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 &amp;quot;\['. 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 \[1967\].</Paragraph>
    <Paragraph position="2"> 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.</Paragraph>
    <Paragraph position="3"> 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.</Paragraph>
    <Paragraph position="4"> For example, the event &amp;quot;the ball moving from x to y&amp;quot; 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 OCCUR(,~ t).</Paragraph>
    <Paragraph position="5"> 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: (forall object, source, goaLt, e)</Paragraph>
    <Paragraph position="7"> HOLDS(at(object, goal), t 2 ) A simple class of events consists of those that occur only if some property remains constant over a particular interval (cPS Jackendoffs STAY verbs). For example, we  t&amp;quot; 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 H O L D S(in( BalI, R oom), T) while the latter sentence describes an event, and hence is of the form REMAIN-IN(Bail, Room, e) &amp; OCCURS(e T).</Paragraph>
    <Paragraph position="8"> We may capture the logical equivalence of the two with the axiom:</Paragraph>
    <Paragraph position="10"> 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.</Paragraph>
    <Paragraph position="11"> 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 &amp;quot;John moved 13 from S to G&amp;quot;</Paragraph>
    <Paragraph position="13"> The axiomadzation for ACAUSE and ALLOW is tricky, but Jackendoff provides a reasonable starting set. In this paper, I shall only consider agency by causation further. The most important axiom about causality is</Paragraph>
    <Paragraph position="15"> For our purposes, one of the most important facts about the ACAUSE relation is that it suggests the possibility of intentionality on the part of the agent. This will be discussed in the next section.</Paragraph>
    <Paragraph position="16"> Note that in this formalism composition of events and actions is trivial. For example, we can define an action composition function together which produces an action or event that consists of two actions or events occuring simultaneously as follows:</Paragraph>
    <Paragraph position="18"/>
  </Section>
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