I am reminded of the professor who, in his declining hours, was asked by his devoted pupils for his final counsel. He replied, Verify your quotations. – Sir Winston Churchill

I am now to offer some thoughts upon that sameness or familiarity which we frequently find between passages in different authors without quotation. This may be one of three things either what is called Plagiarism, or Imitation, or Coincidence. – James Boswell, “The Hypochondriack,” No.XXII, 1779

Next to the originator of a good sentence is the first quoter of it. Many will read the book before one thinks of quoting a passage. As soon as he has done this, that line will be quoted east and west. – Ralph Waldo Emerson, “Quotation and Originality,” Letters and Social Aims, 1876