Aspects of this topic are discussed in the following places at Britannica.
...of appropriate, specific objects, the result is a declarative sentence that is true or false. Given any formula S(x) that contains the letter x (and possibly others), Cantor’s principle of abstraction asserts the existence of a set A such that, for each object x, x ∊ A if and only if S(x) holds. (Mathematicians later...
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