Misattributed and highly interesting:
A god without dominion, providence, and final causes, is nothing else but Fate and Nature.
Isaac Newton: Principia Mathematica (1687); Rules of Reasoning in Philosophy, Rule IV 
A work of art that contains theories is like an object on which the price tag has been left.
Une oeuvre où il y a des théories est comme un objet sur lequel on laisse la marque du prix.
Marcel Proust, In Search of Lost Time, part VII: Time Regained, chapter III, "An Afternoon Party at the House of the Princesse de Guermantes" French versionand English translation
Genius creates, and taste preserves. Taste is the good sense of genius; without taste, genius is only sublime folly.
Le génie enfante, le goût conserve. Le goût est le bon sens du génie; sans le goût, le génie n'est qu'une sublime folie.
François-René de Chateaubriand, Essai sur la littérature anglaise (1836): Modèles classiques 
You beat your pate, and fancy wit will come;Knock as you please, there's nobody at home.
Credited as Epigram: An Empty House (1727), or On a Dull Writer; alternately attributed to Jonathan Swift in John Hawkesworth, The Works of Jonathan Swift, D.D., Dean of St. Patrick's, Dublin (1754), p. 265. Compare: "His wit invites you by his looks to come, But when you knock, it never is at home", William Cowper, Conversation, line 303.
Lull'd in the countless chambers of the brain,Our thoughts are link'd by many a hidden chain.Awake but one, and lo, what myriads rise! Each stamps its image as the other flies!
Samuel Rogers, The Pleasures of Memory (1792), Part I 
Never find fault with the absent.
Absenti nemo non nocuisse velit.
Standard translation: Let no one be willing to speak ill of the absent.
Sextus Propertius, Elegies, II, xix, 32
The hidden harmony is better than the obvious.
Variants: 1) The unapparent connection is more powerful than the apparent one; 2) The hidden harmony is better than the open one.
Heraclitus, Fragments, 54  and 
The sick in body call for aid: the sickIn mind are covetous of more disease;And when at worst, they dream themselves quite well.To know ourselves diseased, is half our cure.
Edward Young, "Night Thoughts," (1742-1745) Part IX 
What some call health, if purchased by perpetual anxiety about diet, isn't much better than tedious disease.
George Dennison Prentice [1807-1870], Prenticeana (1860)
And as Alexander Pope himself would say :
"To err is human, To forgive...divine."