Voltaire was a prolific writer of letters and his correspondence published posthumously by Theodore Besterman consists of 21,221 letters. Of these, more than 15,000 are by Voltaire himself. He met a younger JJ Rousseau in 1745 in Paris and since then these two greats of the Age of Enlightenment continued their correspondence (though not always cordially). In this particular letter Voltaire comments on The Essay against Civilization by Jean-Jaques Rousseau.
August 30, 1755, from Les DELICES. (Voltaire’s home outside Geneva)
I have received, sir, your new book against the human race, and I thank you for it. You will please people by your manner of telling them the truth about themselves, but you will not alter them. The horrors of that human society–from which in our feebleness and ignorance we expect so many consolations–have never been painted in more striking colors: no one has ever been so witty as you are in trying to turn us into brutes: to read your book makes one long to go about all fours. Since, however, it is now some sixty years since I gave up the practice, I feel that it is unfortunately impossible for me to resume it: I leave this natural habit to those more fit for it than are you and I. Nor can I set sail to discover the aborigines of Canada, in the first place because my ill-health ties me to the side of the greatest doctor  in Europe, and I should not find the same professional assistance among the Missouris: and secondly because war is going on in that country, and the example of the civilized nations has made the barbarians almost as wicked as we are ourselves. I must confine myself to being a peaceful savage in the retreat I have chosen–close to your country, where you yourself should be.
I agree with you that science and literature have sometimes done a great deal of harm. Tasso’s  enemies made his life a long series of misfortunes: Galileo’s enemies kept him languishing in prison, at seventy years of age, for the crime of understanding the revolution of the earth: and, what is still more shameful, obliged him to forswear his discovery. Since your friends began the Encyclopedia, their rivals attack them as deists, atheists–even Jansenists.
If I might venture to include myself among those whose works have brought them persecution as their sole recompense, I could tell you of men set on ruining me from the day I produced my tragedy Oedipus… I have no right to complain: Alexander Pope, Descartes, Bayle –a hundred others–have been subjected to the same, or greater, injustice: and my destiny is that of nearly everyone who has loved letters too well.
Confess, sir, that all these things are, after all, but little personal pin-pricks, which society scarcely notices. What matter to humankind that a few drones steal the honey of a few bees? Literary men make a great fuss of their petty quarrels: the rest of the world ignores them, or laughs at them.
They are, perhaps, the least serious of all the ills attendant on human life. …Confess that Italy owed none of her troubles to Petrarch or to Boccaccio: that Marot’s jests were not responsible for the massacre of St. Bartholomew: or Racine’s tragedy. of the Cid for the wars of the Fronde. Great crimes are always committed by great ignoramuses. What makes, and will always make, this world a vale of tears is the insatiable greediness and the indomitable pride of men… Letters support, refine, and comfort the soul: they are serving you, sir, at the very moment you decry them: you are like Achilles declaiming against fame, and Father Malebranche using his brilliant imagination to belittle imagination.
If anyone has a right to complain of letters, I am that person, for in all times and in all places they have led to my being persecuted: still, we must needs love them in spite of the way they are abused–as we cling to society, though the wicked spoil its pleasantness: as we must love our country, though it treats us unjustly: and as we must love and serve the Supreme Being, despite the superstition and fanaticism which too often dishonor His service.
M. Chappus tells me your health  is very unsatisfactory: you must come and recover here in your native place, enjoy its freedom, drink (with me) the milk of its cows, and browse on its grass. I am yours most philosophically and with sincere esteem.
 Dr. Theodore Tronchin, was Voltaire’s doctor from 1754 until Voltaire’s death.
 Torquato Tasso, a famous 16th century Italian court poet.
 Jansenism was a movement within French Catholicism marked for its ultra pious and puritanical views.
 Rousseau suffered throughout his life from a painful condition of the urinary tract, and wrote in excruciating detail about his attempts at cures in his posthumously published Confessions.