The aim of this paper is to analyze Approximate Reasoning (AR) through extensionality with respect to the natural T-indistinguishability operator, by considering the indistinguishability level between fuzzy sets as a formal measure of its degree of similarity, resemblance or closeness, having in all these terms an intuitive meaning.
Boixader, D.; Jacas, J. Extensionality based approximate reasoning. "International journal of approximate reasoning", Juny 1998, vol. 19, núm. 3-4, p. 221-230.