Fuzzy Sets
Zadeh proposed membership in a set as a number between zero and one rather than a yes or no.
Why it matters
A degree of truth gave a way to work with notions like tall or fast without collapsing them to a threshold.
The paper introduces operations on fuzzy sets, convexity and a separation theorem; as fields of application it names pattern recognition, communication of information and abstraction. It contains no control rules of the form if... then.... By the author's account (1980) the idea of graded membership came to him in July 1964. Industrial use came in the 1980s, largely in Japanese control systems. For symbolic AI it was an alternative to two-valued logic as a language for imprecise notions.