Madness is a subject that ought to interest philosophers.
But they have had surprisingly little to say about it.
What they have said, although often interesting and important, has failed to penetrate to the properly philosophical centre of the topic.
They have concerned themselves with its causes and effects, with its social and ethical implications, but they have said little that is useful or definitive about what it is in itself.
Preoccupied with its accidents, they have failed to engage with its essence.
Which is the negation of the cogito, since a schizophrenic cannot argue:
Therefore, I exist.