Full Idea
Principle of Specification: Whenever we can specify a determinate totality of objects, we shall say that there is a set whose elements are precisely the objects that we have specified.
Gist of Idea
Specification: Determinate totals of objects always make a set
Source
José L. Zalabardo (Introduction to the Theory of Logic [2000], §1.3)
Book Reference
Zalabardo,José L.: 'Introduction to the Theory of Logic' [Westview 2000], p.5
A Reaction
Compare the Axiom of Specification. Zalabardo says we may wish to consider sets of which we cannot specify the members.