Full Idea
Socrates began the quest for something universal in addition to the radical flux of perceptible particulars, with his definitions. But he rightly understood that universals cannot be separated from particulars.
Clarification
Plato made definitions separate, with his 'Forms'
Gist of Idea
Socrates began the quest for something universal with his definitions, but he didn't make them separate
Source
report of Socrates (reports of career [c.420 BCE]) by Aristotle - Metaphysics 1086b
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.425