Fragments of a Theory of Human Plausible Reasoning

Fragments of a Theory of Human Plausible Reasoning Allan Collins Bolt Beranek and Newman Inc. ABSTRACT The paper outlines a computational theory of human plausible reasoning constructed from analysis of people's answers to everyday questions. Like logic, the theory is expressed in a content-independent formalism. Unlike logic, the theory specifies how different information in memory affects the certainty of the conclusions drawn. The theory consists of a dimensionalized space of different inference types and their certainty conditions, including a variety of meta-inference types where the inference depends on the person's knowledge about his own knowledge. The protocols from people's answers to questions are analyzed in terms of the different inference types. The paper also discusses how memory is structured in multiple ways to support the different inference types, and how the information found in memory determines which inference types are triggered. INTRODUCTION The goal of this paper is to briefly describe a theory of human plausible reasoning I am currently developing (Collins, 1978). 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 & Collins, 1973; Collins & Warnock, 1974; Collins, Warnock, Aiello & 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. 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. 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. I am trying to construct a deep structure theory from the surface structure traces of the reasoning process. The protocols have the following characteristics. I) There are usually several different inference types used to answer any question. 2) The same inference types recur in many different answers. 3) People weigh all the evidence they find that bears on a question. 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. 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 & 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). (S) Oh right, that is …

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