“Common sense is as rare as genius.”

Lessons : An Autobiography (1986)

"Teaching and Thinking" in The Montreal Medical Journal (1895).

January 29, 1840
Journals (1838-1859)
Context: Aeschylus had a clear eye for the commonest things. His genius was only an enlarged common sense. He adverts with chaste severity to all natural facts. His sublimity is Greek sincerity and simpleness, naked wonder which mythology had not helped to explain... Whatever the common eye sees at all and expresses as best it may, he sees uncommonly and describes with rare completeness. The multitude that thronged the theatre could no doubt go along with him to the end... The social condition of genius is the same in all ages. Aeschylus was undoubtedly alone and without sympathy in his simple reverence for the mystery of the universe.

The Principles of Success in Literature (1865)
Context: Genius is rarely able to give any account of its own processes. But those who have had ample opportunities of intimately knowing the growth of works in the minds of artists, will bear me out in saying that a vivid memory supplies the elements from a thousand different sources, most of which are quite beyond the power of localisation, the experience of yesterday being strangely intermingled with the dim suggestions of early years, the tones heard in childhood sounding through the diapason of sorrowing maturity; and all these kaleidoscopic fragments are recomposed into images that seem to have a corresponding reality of their own.

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, in "Essai sur la littérature anglaise (1836): Modèles classiques http://visualiseur.bnf.fr/CadresFenetre?O=NUMM-101390&M=tdm.
Misattributed

Lloyd George is portrayed as saying this, as George Nathaniel Curzon was making a complaint against Raymond Poincaré in the Turkish TV series, Kurtuluş (1994), but no prior citation of such a statement has yet been found.
Misattributed

Source: Darkly Dreaming Dexter