is the two-element set holding the values true and false, a subset S is that subset for which the predicate holds, and which is true if is not empty, and which is false if S is not X. The universal and existential quantifiers given above generalize to the presheaf category. See also Existential quantification First-order logic List of logic symbols—for the Unicode symbol ∀ Notes References (ch. 2) External links Logic symbols Logical expressions Quantifier (logic)