“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