What's interesting about Shakespeare is that he's interested in madness as a language. What Shakespeare is saying is […] that mad people speak in an extraordinary way. And one of the things about mad people is that they seem able to perform being mad as well as as it were being mad or we wonder really what they want us to think they're saying. 

They perplex us […] Mad people are very important to Shakespeare, because it is as though they enact how perplexing language is, especially when it's as its most intensely poetic.

Adam Phillips, from a South Bank Special on Art and Insanity here.