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A Word About the Logic of MYCIN 6520. HERSTORYList: Another addition to the rule monitor in MYCINwas a mechanism for keeping track of all rules invoked, failing, succeeding,etc., and the reasons for these various outcomes. The so-called HERS-TORY List, or history tree, then provided the basis for MYCIN’s explanationsin response to users’ queries.21. Creation of a Patient Data Table: Finally, we recognized the need todevelop mechanisms for (a) reevaluating cases when more informationbecame available and (b) assessing the impact of modifications to the knowledgebase on a library of cases previously handled well. These goals wereachieved by the development of a Patient Data Table, i.e., a mechanism forstoring and accessing the initializing conditions necessary for full considerationof cases. See Chapter 5 for further discussions of this feature.3.3 A Word About the Logic of MYCINThe logic of MYCIN’s reasoning is propositional logic, where the elementarypropositions are fact triples and the primary rule of inference is modusponens (A and A D B implies B). It is extended (and somewhat complicated)in the following respects:¯ Certainty factors (CF’s) are attached (or propagated) to all propositions.¯ CF’s are associated with all implications.¯ Predicates are associated with fact triples to change the way facts statedin rules are matched against facts in the dynamically constructed caserecord. A variety of predicates have been defined (see Section 5.1.5);some refer to values of attributes (e.g., NOT-SAME, ONE-OF) and somereference values of CF’s (e.g., KNOWN, DEFINITE).¯ Limited quantification is allowed over conjunctions of propositions (e.g.,THERE-IS, FOR-EACH).¯ Meta-level reasoning is allowed in order to increase efficiency (e.g., usingmeta-rules or looking for a unity path).MYCIN’s logic is incomplete in the sense that we know there are propositionsthat can be expressed in the language but are not provable astheorems. MYCIN’s logic is not inconsistent in itself (we believe), but itnot immune to inconsistencies introduced into its knowledge base.
66 The Evolution of MYCIN’s Rule Form3,4Overview of Part TwoThe remainder of this part consists of three papers that summarize MY-CIN and its use of production rules. In order to orient the reader toMYCIN’s overall motivation and design, we first include as Chapter 4 anintroductory paper that provides an overview of the system as of 1978(approximately the time when development of the medical knowledge basestopped). Chapter 5 is the original detailed description of MYCIN from1975. It provides technical information on the system’s representation andcontrol mechanisms. Chapter 6 is a brief paper from 1977 that discussesthe way in which production rules were adapted to deal with the algorithmicknowledge regarding therapy selection.
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