Full Idea
Principle of Determinacy: For every object a and every set S, either a is an element of S or a is not an element of S.
Gist of Idea
Determinacy: an object is either in a set, or it isn't
Source
José L. Zalabardo (Introduction to the Theory of Logic [2000], §1.2)
Book Reference
Zalabardo,José L.: 'Introduction to the Theory of Logic' [Westview 2000], p.4