In the New York Times of March 6, 2010, Pat Ryan wrote:
“Is the Hatter mad?
Since 1865, when Alice in Wonderland was published, readers have quoted and parsed his every utterance. He’s called simply the Hatter in Alice and Hatta in Through the Looking-Glass, but we know he’s mad; the Cheshire-Cat tells us so.
…The phrase “he’s mad as a hatter” was colloquial in Britain before Alice*. Inquiries “respecting this simile” had appeared in the journal Notes & Queries, and in 1863 an answer, of sorts, was published…Readers were referred to the French phrase “Il raisonne comme une huitre” (“He reasons like an oyster”), suggesting that the French word for oyster, when Anglicized, may “have given occasion to the English ‘hatter.’ ” Hmm.
…Linguists and phraseologists have tried to pinpoint the simile’s origins. Eric Partridge, in his Dictionary of Slang and Unconventional English, concluded that Carroll “definitely fixed the English sense” of the phrase as “extremely eccentric.”
…Since Carroll was well known as a mathematician, logicians have looked to his writing and discovered a sympathetic mind under the Hatter hat: perhaps a Mad Adder**.
….Next we come to the notorious “hatters’ shakes,” a result of poisoning from mercury used in the early days of hat manufacturing….In the British Medical Journal in 1983, however, H.A. Waldron concluded that the Hatter did not have mercury poisoning. The principal psychotic features of this type of poisoning are “excessive timidity, diffidence, increasing shyness, loss of self confidence, anxiety and a desire to remain unobserved and unobtrusive.” The Hatter, he states, was “an eccentric extravert.”
All of this fits the spirit of illogicism in Alice. In response to one textual query, Carroll answered: “I’m very much afraid I didn’t mean anything but nonsense! Still, you know, words mean more than we mean to express when we use them.” He would have liked the Unreasoning Oyster.”.
(From Wikipedia: *The earliest known appearance of the phrase in print is in an 1829 issue of Blackwood’s Edinburgh Magazine.
**According to A Dictionary of Common Fallacies (1980), “‘mad’ meant ‘venomous’ and ‘hatter’ is a corruption of ‘adder’, or viper, so that the phrase ‘mad as an atter’ originally meant ‘as venomous as a viper’.”)
However: the title of Chapter Seven of Alice is “A Mad Tea-Party”; and in the last line of Chapter Six, Alice is referring to the March Hare when she says:
“Suppose it should be raving mad after all! I almost wish I’d gone to see the Hatter instead!’.
Paul Brown wrote in The Guardian in March 2013:
“Despite the reduction in their numbers one of the most delightful signs of spring remains the sight of hares leaping in the air, apparently engaged in a boxing match. Once seen it is obvious why “Mad as a March hare” is an expression the British have used for 600 years.
The European hare, Lepus europaeus, is taking part in a mating ritual of sorts. It is usually the female landing the blows, fending off over amorous males. Observers have also thought this behaviour is also aimed at testing the male’s strength before deciding whether to proceed on the next step of courtship. Either way in the still bare March fields this is the best opportunity to view these otherwise mainly nocturnal animals.”.
Chapter Six: “How do you know I’m mad?” said Alice.
“You must be,” said the Cat, “or you wouldn’t have come here.” —