The Hatter and the March Hare are described as "both mad" by the Cheshire Cat, in Alice's Adventures in Wonderland in the sixth chapter titled "Pig and Pepper".

The Hatter character, alongside all the other fictional beings, first appears in Lewis Carroll's 1865 novel Alice's Adventures in Wonderland.

[1] Mercury poisoning causes neurological damage, including slurred speech, memory loss, and tremors, which led to the phrase "mad as a hatter".

[1] In the Victorian age, many workers in the textile industry, including hatters, sometimes developed illnesses affecting the nervous system, such as central nervous system (CNS) tuberculosis, which is portrayed in novels like Alton Locke by Charles Kingsley and North and South by Elizabeth Gaskell, which Lewis Carroll had read.

Many such workers were sent to Pauper Lunatic Asylums, which were supervised by Lunacy Commissioners such as Samuel Gaskell and Robert Wilfred Skeffington Lutwidge, Carroll's uncle.

[4] The Hatter and his tea party friend, the March Hare, are initially referred to as "both mad" by the distinctive Cheshire Cat.

Hat making was the main trade in Stockport where Carroll grew up, and it was not unusual then for hatters to appear disturbed or confused; many died early as a result of mercury poisoning.

However, the Hatter does not exhibit the symptoms of mercury poisoning, which include excessive timidity, diffidence, increasing shyness, loss of self-confidence, anxiety, and a desire to remain unobserved and unobtrusive.

The April 2017 edition of Bandersnatch, the Newsletter of the Lewis Carroll Society [Issue 172, ISSN 0306-8404, Apr 2017], published the following solution, proposed by puzzle expert Rick Hosburn: "Why is a Raven like a Writing-desk?"

Snowbound and isolated "ten thousand feet high" in the Rocky Mountains, five-year-old Danny hears whispers of the malign "voice of the [Overlook] hotel" inside his head, including this bit of mockery: "Why is a raven like a writing desk?

The character has been portrayed in film by Norman Whitten, Edward Everett Horton, Sir Robert Helpmann, Martin Short, Peter Cook, Anthony Newley, Ed Wynn, Andrew-Lee Potts, and Johnny Depp.

He has also been portrayed on stage by Nikki Snelson and Katherine Shindle, and on television by John Robert Hoffman, Pip Donaghy and Sebastian Stan.