It is easy to say we feel the mediocrity when we have heard a good many people say that the work is mediocre, but, unless in matters about which he has been long conversant, no man can easily form an independent judgment as to whether or not a work is mediocre.
 Samuel Butler, The Note-Books of Samuel Butler (1912). copy citation

Context

“But I suppose this really is attending to style after all. [1897.] Diderot on Criticism “Il est si difficile de produire une chose même médiocre; il est si facile de sentir la médiocrité.” I have lately seen this quoted as having been said by Diderot. It is easy to say we feel the mediocrity when we have heard a good many people say that the work is mediocre, but, unless in matters about which he has been long conversant, no man can easily form an independent judgment as to whether or not a work is mediocre. I know that in the matter of books, painting and music I constantly find myself unable to form a settled opinion till I have heard what many men of varied tastes have to say, and have also made myself acquainted with details about a man’s antecedents and ways of life which are generally held to be irrelevant.” source