The polis gives each individual his due place in its political cosmos, and thereby gives him, besides his private life, a sort of second life, his βίος πολιτικός. Now, every citizen belongs to two order of existence; and there is a sharp distinction in his life between what is his own (ἴδιον) and what is communal (κοινόν). Man is not only ‘idiotic,’ he is also ‘politic.’ As well as his ability in his own profession or trade, he has his share of the universal ability of the citizen, πολιτικὴ ἀρετή, by which he is fitted to co-operate and sympathize with the rest of the citizens in the life of the polis

☛ Paideia: the Ideals of Greek Culture by Werner Jaeger, tr. by Gilbert Highet, New York: Oxford University Press, volume I, [1933]1945, p. 111

I first found (a shorter version of) this quote while reading Hannah Arendt’s The Human Condition (Chicago: The University of Chicago Press, [1958]1998, p. 24: it’s part of the chapter on “The Public and the Private Realm”). There’s (what I assume to be) a small typo in both the French and the English edition of that book: the quote by Jaeger is attributed to volume III, but it actually comes from volume I.
In a footnote, Jaeger develop the meaning of “idiotic” and its relationship with the political life (see p. 444, note 45):

The Greek word ἰδιώτης means the opposite of πολίτης, even the same individual is both a private person and a member of the political community (τά ἴδια opp. τά δημόσια). The contrast is intensified when the ἰδιώτης is compared with the actual politician (πολιτικός) or with any person whose life is devoted to some kind of public service (δημιουργός), such as craftsmen. In that connexion, ἰδιώτης comes to mean a layman. In Plato’s language , e.g. in the Republic, ἰδιώτης means the individual, so far as he has no influence on public opinion and life. […]

Compare with Hölderlin’s poem Bread and Wine:

One thing is sure even now: at noon or just before midnight,
Whether it’s early or late, always a mesure exists,
Common to all, though his own to each one is also alloted,
Each of us makes for the place, reaches the place that he can.

See Poems and fragments by Friedrich Hölderlin, tr. by Michael Hamburger, New York: Cambridge University Press, 1980, p. 245 (available online: PDF). Those verses are used for the opening exergue of the third French edition of La Communauté désoeuvrée by Jean-Luc Nancy (Paris: Christian Bourgois, [1986]1999). They do not exist in the English translation which was actually published eight years earlier (The Inoperative Community, Minneapolis: University of Minnesota Press, [1986]1991).