* Defining "essay" is subjective. There's no strict length limit for what constitutes an essay, and some works may be labeled as essays even if they are quite lengthy.
* Records are not meticulously kept. There's no central database tracking the lengths of all essays ever written.
* Length isn't the only measure of importance. Some incredibly important and influential essays are relatively short, while some very long works might not be widely read or considered significant.
That being said, some contenders for the title of "longest essay" include:
* "The History of the Peloponnesian War" by Thucydides: This ancient Greek work, written in the 5th century BC, is considered by many to be the first true historical essay. It's about 300 pages long in a modern English translation.
* "The Decline and Fall of the Roman Empire" by Edward Gibbon: This massive six-volume work, published in the late 18th century, is a monumental study of the Roman Empire's decline and fall. It's over 4,000 pages long.
* "The Life of Samuel Johnson" by James Boswell: This biographical work, published in 1791, is a massive and detailed account of the life of the famous English writer and lexicographer Samuel Johnson. It's over 1,000 pages long.
Ultimately, the "longest essay ever written" is a matter of opinion and definition. The works listed above are all massive and significant in their own right, and their lengths are a testament to the ambitious scope of their topics.