Perspectives of Madness in Twelfth Night

Shakespeare uses such words as 'mad' and 'madness' more often in Twelfth Night than in any of his other plays, so it is a reasonable assumption that he was interested in madness when he wrote it, and that this play will give us an idea of what he means by it. Interestingly, he never gives us a definition of the word 'madness' per se. As in other plays, we are left to work out the definition for ourselves, and the dramatist rather behaves as though the meaning is readily understood by all members of the audience. What, in fact, did Shakespeare mean by the term