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 and the basis for rules of the form if the temperature is high, reduce the flow. Industrial use came in the 1980s, largely in Japanese control systems. For symbolic AI it was the first serious alternative to two-valued logic as a knowledge representation language.