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<Paper uid="T78-1028">
  <Title>Fragments of a Theory of Human Plausible Reasoning</Title>
  <Section position="2" start_page="0" end_page="194" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> The goal of this paper is to briefly describe a theory of human plausible reasoning I am currently developing (Collins, 1978).</Paragraph>
    <Paragraph position="1"> The theory is a procedural theory and hence one which can be implemented in a computer, as parts of it have been in the SCHOLAR and MAP-SCHOLAR systems (Carbonell &amp; Collins, 1973; Collins &amp; Warnock, 1974; Collins, Warnock, Aiello &amp; Miller, 1975). The theory is expressed in the production-rule formalism of Newell (1973). Unlike logic, the theory specifies how different configurations of information affect the certainty of the conclusions drawn. These certainty conditions are in fact the major contribution of the theory.</Paragraph>
    <Paragraph position="2"> Methodology of Constructing the Theory To construct a theory of human plausible reasoning, I collected about 60 answers to everyday questions from 4 different subjects. The questions ranged from whether there are black princess phones to when the respondent first drank beer.</Paragraph>
    <Paragraph position="3"> The analysis of the protocols attempts to account for the reasoning and the conclusions drawn in the protocols in terms of: I) a taxonomy of plausible inference types, 2) a taxonomy of default assumptions, and 3) what the subject must have known a priori. As will be evident, this is an inferential analysis.</Paragraph>
    <Paragraph position="4"> I am trying to construct a deep structure theory from the surface structure traces of the reasoning process.</Paragraph>
    <Paragraph position="5"> The protocols have the following characteristics.</Paragraph>
    <Paragraph position="6"> I) There are usually several different inference types used to answer any  question.</Paragraph>
    <Paragraph position="7"> 2) The same inference types recur in many different answers.</Paragraph>
    <Paragraph position="8"> 3) People weigh all the evidence they find that bears on a question.</Paragraph>
    <Paragraph position="9"> 4) People are more or less certain depending on the certainty of the information, the  certainty of the inferences, and on whether different inferences lead to the same or opposite conclusions.</Paragraph>
    <Paragraph position="10"> I can illustrate some of these characteristics of the protocols as well as several of the inference types in the theory with a protocol taken from a tutorial session  on South American geography (Carbonell &amp; Collins, 1973): (T) There is some jungle in here (points to Venezuela) but this breaks into a savanna around the Orinoco (points to the Llanos in Venezuela and Colombia).</Paragraph>
    <Paragraph position="11"> (S) Oh right, that is where they grow the  coffee up there? (T) I don't think that the savanna is used for growing coffee. The trouble is the savanna has a rainy season and you can't count on rain in general. But I don't know. This area around Sao Paulo (in Brazil) is coffee region, and it is sort of getting into the savanna region there. In the protocol the tutor went through the following reasoning on the question of whether coffee is grown in the Llanos.</Paragraph>
    <Paragraph position="12"> Initially, the tutor made a hedged &amp;quot;no&amp;quot;  response for two reasons. First, the tutor did not have stored that the Llanos was used for growing coffee. Second, the tutor knew that coffee growing depends on a number of factors (e.g., rainfall, temperature, soil, and terrain), and that savannas do not have the correct value for growing coffee on at least one of those factors (i.e., reliable rainfall). However, the tutor later hedged his initial negative response, because he found some positive evidence. In particular, he thought the Brazilian savanna might overlap the coffee growing region in Brazil around Sao Paulo and that the Brazilian savanna might produce coffee. Thus by analogy the Llanos might also produce coffee. Hence, the tutor ended up saying &amp;quot;I don't know.&amp;quot; The answer exhibits a number of the important aspects of the protocols. In general, a number of inferences are used to derive an answer. Some of these are inference chains where the premise of one inference depends on the conclusion of another inference. In other cases the inferences are independent sources of evidence. When there are different sources of evidence, the subject weighs them together to determine his conclusion.</Paragraph>
    <Paragraph position="13"> It is also apparent in this protocol how different pieces of information are found over time. What appears to happen is that the subject launches a search for relevant information (Collins &amp; Loftus, 1975). As relevant pieces of information are found (or are found to be missing), they trigger particular inferences. The type of inferenoe applied is determined by the relation between the information found and the question asked. For example, if the subject knew that savannas are in general good for growing coffee, that would trigger a deduction. If the subject knew of one savanna somewhere that produced coffee, that would trigger an analogy. The search for information is such that the most relevant information is found first. In the protocol, the more relevant information about the unreliable rainfall in savannas was found before the more far fetched information about the coffee growing region in Brazil and its relation to the Brazilian savanna. Thus, information seems to be found at different times by an autonomous search process, and the particular information found determines inferences that are triggered.</Paragraph>
  </Section>
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