axiom of extensionality

(from the article `mathematics, foundations of`) ...asserts that, for every (formula or statement), there should exist a set such that, for all , if and only if () is true. Moreover, by the ... ...with each other, even if they are specified by different conditions; i.e., identity of classes is identity of membership, not identity o...
Found on http://www.britannica.com/eb/a-z/a/132
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