“Be what you would seem to be.”
English proverb, used by many authors, including some prior to Margaret Fuller's time; Thomas Fuller expresses related thoughts in his "Panegyric" on Charles II, Section 21" in The History of the Worthies of England (1662):
Be you above your ancestors renown'd,
Whose goodness wisely doth your greatness bound;
And, knowing that you may be what you would,
Are pleased to be only what you should.
Misattributed
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Margaret Fuller 116
American feminist, poet, author, and activist 1810–1850Related quotes

“849. Be as you would seem to be.”
Compare Poor Richard's Almanack (1744) : What you would seem to be, be really.
Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

Discussing the influence of real-life faiths on his work and its religious systems, Authors@Google (August 2011)
Context: I think worship of death is an interesting basis for religion, because after all death is the one universal. It doesn't seem to matter what gods you pray to. We all die, in the real world and in fantasy worlds, and if there was some religion where you did not die I suspect that would be, that god would become very popular. They all promise us eternal life, but whatever.

Source: Thorns (1967), Chapter 21, “And Southward Aye We Fled” (p. 106)

Source: Looking Backward, 2000-1887 http://www.gutenberg.org/ebooks/25439 (1888), Ch. 26.

“T is what you will,—or will be what you would.”
First Week, Third Day.
La Semaine; ou, Création du monde (1578)