Outline of a Theory of Usuality Based on Fuzzy Logic

  • L. A. Zadeh
Part of the NATO ASI Series book series (ASIC, volume 177)


The concept of usuality relates to propositions which are usually true or, more precisely, to events which have a high probability of occurrence. For example, usually Cait is very cheerful, usually a TV set weighs about fifty pounds, etc. Such propositions are said to be usuality-qualified. A usuality-qualified proposition may be expressed in the form usually (X is F), in which X is a variable taking values in a universe of discourse U and F is a fuzzy subset of U which may be interpreted as a usual value of X. In general, a usual value of variable, X, is not unique, and any fuzzy subset of U qualifies to a degree to be a usual value of X. A usuality qualified proposition in which usually is implicit rather than explicit is said to be a disposition. Simple examples of dispositions are snow is white, a cup of coffee costs about fifty cents and Swedes are taller than Italians.

In this paper, we outline a theory of usuality in which the point of departure is a method of representing the meaning of usuality-qualified propositions. Based on this method, a system of inference for usuality-qualified propositions may be developed. As examples, a dispositional version of the Aristotelian Barbara syllogism as well as a dispositional version of the modus ponens are described. Such dispositional rules of inference are of direct relevance to commonsense reasoning and, in particular, to commonsense decision analysis. A potentially important application area for the theory of usuality is the management of uncertainty in expert systems.


Membership Function Fuzzy Logic Fuzzy Number Fuzzy Subset USUALITY Base 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© D. Reidel Publishing Company 1986

Authors and Affiliations

  • L. A. Zadeh
    • 1
  1. 1.University of CaliforniaBerkeleyUSA

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