The Boolean domain {0, 1} can be replaced by the unit interval [0,1], in which case rather than only taking values 0 or 1, any value between and including 0 and 1 can be assumed. Algebraically, negation (NOT) is replaced with 1 − x , {\displaystyle 1-x,} conjunction (AND) is replaced with multiplication ( x y {\displaystyle xy} ), and disjunction (OR) is defined via De Morgan's law to be 1 − ( 1 − x ) ( 1 − y ) = x + y − x y {\displaystyle 1-(1-x)(1-y)=x+y-xy} .
Interpreting these values as logical truth values yields a multi-valued logic, which forms the basis for fuzzy logic and probabilistic logic. In these interpretations, a value is interpreted as the "degree" of truth – to what extent a proposition is true, or the probability that the proposition is true.
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