Full Idea
Universal Specification: from ∀xP(x) we may conclude P(t), where t is an appropriate term. If something is true for all members of a domain, then it is true for some particular one that we specify.
Gist of Idea
Universal Specification: ∀xP(x) implies P(t). True for all? Then true for an instance
Source
Robert S. Wolf (A Tour through Mathematical Logic [2005], 1.3)
Book Reference
Wolf,Robert S.: 'A Tour Through Mathematical Logic' [Carus Maths Monographs 2005], p.20