"The Poetic Principle" is an essay by Edgar Allan Poe, written near the end of his life and published posthumously in 1850, the year after his death.
He specifically targeted Henry Wadsworth Longfellow for his didacticism, something he would go on to call "the heresy of the didactic".
[1] The essay was based on a lecture that Poe gave in Providence, Rhode Island at the Franklin Lyceum.
From this experience, Poe surmised that long poems are unable to sustain a proper mood or maintain a high-quality poetic form and are, therefore, inherently flawed.
[3] Critics have suggested that this theory was written so that Poe could justify why "Al Aaraaf" and "Tamerlane" were unpopular.