A hatter, well that's the poor sop who used to make hats, felt hats, specifically. The best sorts of felt hats required fur from beaver or rabbit pelts. And, of course, the fur from the pelts need processing in order to become a hat. By processing, of course we mean the addition of some kind of unfriendly chemicals. (Are there any other kind of chemicals, but the unfriendly kind, i ask you, dear reader?) Mercury applied to the fur made it rougher, matted ~ you know, to harden the fur so the hatter could shape it, iron it, steam it into its finished form, usually a top hat.
Hatters typically worked in poorly ventilated areas, and so the fumes from the mercury compound literally went to their heads. Ahhh, the pleasantries of mercury poisoning: brain damage ... kidney damage ... a painful death, sooner rather than later. What does that look and feel like? Like this ~ yellowing of skin, intense itchiness, loosening of teeth, mouth sores, bleeding gums, loss of co-ordination, slurred speech, and personality changes such as irritability, paranoia, memory loss, depression, anxiety, abdominal cramping, breathing difficulties, cardiac malfunction, muscle cramps. You get the idea, right? Sounds charming, huh? [NOT]
A so, now you know what Mad as a Hatter means. Makes one value sanity, doesn't it? Indeed, it does. It reminds one, too, of the pain of madness. Yes, it hurts when one is mad ... insane ... unstable ... mentally ill. Make no bones about it.
This means challenging my self to compassionate, encouraging, genuine and hostile-free exchanges. It also means contemplating my reaction to the thoughts, feelings and expressions of others, prior to acting upon them. 
