All quotation dictionaries stand on the shoulders of their predecessors, which must be consulted as part of the effort to make sure that no famous quotations are missed. – Fred R. Shapiro, The Yale Book of Quotations, 2006 (Acknowledgements)

Besides, it happens (how, I cannot tell) that an idea launched like a javelin in proverbial form strikes with sharper point on the hearer’s mind and leaves implanted barbs for meditation… – Desiderius Erasmus, Adages

An apt quotation is as good as an original remark. – Author Unknown

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