Sourced

• Simplicity is the most deceitful mistress that ever betrayed man.
  • Henry Adams, The Education of Henry Adams (1907)

• 'Tis the gift to be simple, 'tis the gift to be free.
  • Joseph Brackett, Simple Gifts (1848)

• Numquam ponenda est pluralitas sine necessitate.
  • Plurality ought never be posited without necessity.
  • William of Occam, Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi (ed. Lugd., 1495), i, dist. 27, qu. 2, K
  • Commonly paraphrased as Occam's razor:
    • Entia non sunt multiplicanda praeter necessitatem.
      • Entities should not be multiplied beyond necessity.

• If you can’t reduce a difficult engineering problem to just one 8-1/2 x 11-inch sheet of paper, you will probably never understand it.
  • Ralph Brazelton Peck, as quoted by

• Everything should be made as simple as possible, but no simpler.
  • Albert Einstein, On the Method of Theoretical Physics (1933)

Dictionary of Burning Words of Brilliant Writers (1895)

• Nothing is more simple than greatness; indeed to be simple is to be great.
  • Ralph Waldo Emerson, p. 544.

• "Blessed are the poor in spirit." Blessed are they who are stripped of every thing, even of their own wills, that they may no longer belong to themselves.
  • François Fénelon, p. 544.

• God would behold in you a simplicity which will contain so much the more of His wisdom as it contains less of your own.
  • François Fénelon, p. 544.

• True simplicity regards God alone; it has its eye fixed upon Him, and is not drawn toward self; and it is as pleased to say humble as great things. All our uneasy feelings and reflections arise from self-love, whatever appearance of piety they may assume. The lack of simplicity inflicts many wounds. Go where we will, if we remain in ourselves, we shall carry everywhere our sins and our distresses. If we would live in peace, we must lose sight of self, and rest in the infinite and unchangeable God.
  • Madame Guyon, p. 544.

• He sows June fields with clover, and the world
  Broadcasts with little common kindnesses.
  The plain good souls He sends us, who fulfill
  Life's homely duties in the daily path
  With cheerful heart, ambitious of no more
  Than to supply the wants of friend and kin,
  Yet serve God's higher love to human hearts;
  Giving a secret sweetness to the home,
  The hidden fragrance of a kindly heart,
  The simple beauty of a useful life,
  That never dazzles, and that never tires.
  • Samuel Longfellow, p. 544.

• Simpler manners, purer lives; more self-denial; more earnest sympathy with the classes that lie below us, nothing short of that can lay the foundations of the Christianity which is to be hereafter, deep and broad.
  • Frederick William Robertson, p. 545.

• Simplicity and purity are the two wings by which a man is lifted above all earthly things. Simplicity is in the intention — purity in the affection. Simplicity tends to God,— purity apprehends and tastes Him.
  • Thomas à Kempis, p. 545.

• As to our friend, I pray God to bestow upon him a simplicity that shall give him peace. Happy are they indeed who can bear their sufferings in the enjoyment of this simple peace and perfect acquiesence in the will of God.
  • François Fénelon, p. 545.

• If our love were but more simple,
  We should take Him at His word;
  And our lives would be all sunshine
  In the sweetness of the Lord.
  • Frederick William Faber, p. 545.

• If you wish to be like a little child, study what a little child could understand — nature; and do what a little child could do — love.
  • Charles Kingsley, p. 545.

Unsourced

• Our life is frittered away by detail.... Simplify, simplify. Henry David Thoreau

• Simplicity through breadth. Neils Bohr

• Perfection is attained, not when no more can be added, but when no more can be removed. Antoine de Saint Exupéry

• Simplicity is the ultimate sophistication. ~ Leonardo Da Vinci

• I would have written a shorter letter, but I did not have the time. —Blaise Pascal

This quote has been also attributed to Mark Twain, T.S. Eliot, Cicero, and others besides.