Full Idea
Just as the foot in detachment is no longer a foot, so you in detachment are not longer a man. For what is a man? A part of a city, first.
Gist of Idea
A person is as naturally a part of a city as a foot is part of the body
Source
Epictetus (The Discourses [c.56], 2.05.26)
Book Reference
Epictetus: 'The Discourses, The Handbook, Fragments', ed/tr. Gill,C [Everyman 1995], p.85
A Reaction
It is, of course, not true that a detached foot ceases to be a foot (and an isolated human is still a human). This an extreme version of the Aristotelian idea that we are essentially social. It is, though, the sort of view favoured by totalitarianism.