Plagiarism and cryptomnesia
Cryptomnesia (literally "hidden memory") is a situation in which a latent memory is misidentified as one's original idea. When that idea is used in an original work without attribution, the user has inadvertently committed plagiarism.
Two forms of cryptomnesia have been recognized by researchers. These may be referred to as the forgotten thought and the forgotten source .
The Forgotten Thought
When a thought has been entirely forgotten for a period of time and then is suddenly remembered, the plagiarist has no logical choice but to assume it to be his original idea. This is the form of memory bias which may have influenced Lord Byron in the writing of his tragedy Manfred.
Readers of Manfred took note of the striking similarities of that work to Goethe's Faustus, and Goethe himself was very complimentary of Byron's erudition in adapting Goethe's work to his own purposes. Byron, however, claimed never to have read Faustus.
The Forgotten Source
In this form of cryptomnesia, the idea itself is correctly remembered to be of earlier origin, but because its source is forgotten, the plagiarist mistakenly assumes himself to be the originator of the idea. Robert Louis Stevenson, in The Art of Writing, confessed that much of the descriptive detail in the early chapters of his Treasure Island was "the property of Washington Irving." This he discovered several years later, to his chagrin, when he revisited a copy of Irving's Tales of a Traveller. As Stevenson describes the circumstances, "...I had no guess of it then as I sat writing by the fireside, in what seemed the spring-tides of a somewhat pedestrian inspiration...It seemed to me original as sin; it seemed to belong to me like my right eye..."
Cryptomnesia is No Excuse
Copyright law in the United States is unmoved by claims of cryptomnesia as a defense to charges of plagiarism. In 1976, former Beatle George Harrison was sued for copyright infringement by the publishers of "He's So Fine," a song written in 1962 by Ronald Mack. The court found that Harrison's song "My Sweet Lord" borrowed substantially from "He's So Fine," and held Harrison liable for damages despite Harrison's claim that the plagiarism was unknowing and unintentional.