Kleene's Three-Valued Logic

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This Demonstration presents a simple test for Kleene's three-valued logic. The values are: True, False, and Undefined (or Unknown or Undecided), with associated numerical values 1, 0, 1/2, respectively. The value of compound propositions is evaluated according to Kleene's truth tables.


A simple two-dimensional area is occupied by triangles, squares, and pentagons of three sizes and two colors. A disk-shaped figure means that the shape of the element is not known. In this case a proposition of type Shape() has value "undefined".


Contributed by: Izidor Hafner (March 2011)
Based on work by: Jon Barwise and John Etchemendy
Open content licensed under CC BY-NC-SA



J. Barwise and J. Etchemendy, Tarski's World, Stanford, CA: CSLI Publications, 1993.

J. Barwise and J. Etchemendy, Hyperproof, Stanford, CA: CSLI Publications, 1994, pp. 11–13.

S. C. Kleene, Introduction to Matamathematics, New York: Elsevier, 1952, pp. 332–335.

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