[quote=solamelike]
The joker being mentally ill has been up for debateThe joker is apparently not insane as he knows everything that he is doing is wrong and he just plays insanity so he goes to Arkham asylum so he can escape. All his lawyers also help him plead insanity. Only the joker and Batman know that he isn't actually insane he is just twisted.
[/quote]

It kind of depends on how you define insanity. The Joker, in most (if not all) incarnations, does show symptoms for several mental illnesses and personality disorders.