We sometimes think of quotations as extracts from larger texts, but some quotations originated complete unto themselves. – Gary Saul Morson, The Words of Others: From Quotations to Culture, 2011

We prefer to think that the absence of inverted commas guarantees the originality of a thought, whereas it may be merely that the utterer has forgotten its source. – Clifton Fadiman, The American Treasury, 1455-1955, 1955

He picked something out of everything he read. – Pliny

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