“Art is the sanctification of the nature, of that nature found in everyone who is content to live.”

Proem
Festus (1839)

Source: To the Memory of My Beloved, the Author, Mr. William Shakespeare (1618), Lines 55 - 70
Context: Yet must I not give nature all: thy art,
My gentle Shakspeare, must enjoy a part.
For though the poet's matter nature be,
His art doth give the fashion. And that he
Who casts to write a living line, must sweat,
(Such as thine arc) and strike the second heat
Upon the muses anvil; turn the fame,
And himself with it, that he thinks to frame;
Or for the laurel, he may gain a scorn,
For a good poet's made, as well as born.
And such wert thou. Look how the father's face
Lives in his issue, even so the race
Of Shakspeare's mind and manners brightly shines
In his well-turned, and true filed lines:
In each of which he seems to shake a lance,
As brandish'd at the eyes of ignorance.

XI, 10
Meditations (c. 121–180 AD), Book XI

Meditations. xi. 10.
Bartlett's Familiar Quotations, 10th ed. (1919)

Small Houses: Their Economic Design and Construction (1922)
Context: Why can there not be a new art founded on the only principle which can produce great art—the principle that art is the interpretation or extraction of the essence of beauty in nature, and all else is secondary?<!-- Introduction

to Euphrates, Epp. Apoll. 18
Letters

As reported by Charles Simmons in A Laconic Manual and Brief Remarker, containing over a thousand subjects alphabetically and systematically arranged (North Wrentham, Mass. 1852), p. 103 http://books.google.de/books?id=YOAyAQAAMAAJ&pg=PA103&dq=socrates. However, the original source of this statement is unknown.

Cf. Joseph Addison in The Spectator No. 574 Friday, July 30, 1714, p. 655 http://books.google.de/books?id=K1cdAAAAMAAJ&pg=PA655&dq=socrates: In short, content is equivalent to wealth, and luxury to poverty; or, to give the thought a more agreeable turn, "content is natural wealth," says Socrates: to which I shall add, "luxury is artificial poverty.".
Attributed

Nature and Art http://www.newmanreader.org/works/verses/verse5.html, st. 12 (1868).

Mastery http://www.poemhunter.com/poem/mastery-2/
From the poems written in English