When I first read Boswell's Life of Johnson it never occurred to me that the oddities of Johnson's behaviour were anything other than the quirks of a great man and of no special interest. Since then on more than one occasion I've come across the diagnosis/thesis that he was bi-polar and that understanding this helps explain him.
Reading the Life now this seems to be obviously the case. Indeed, Boswell is clearly aware of the problem, or, rather, of some kind of problem associated with a kind of melancholia that he can't quite describe or put a name to.
The strange thing is that far from reducing Johnson in stature an awareness - perhaps in a somewhat simplified form - of the syndrome he's dealing with serves to enhance one's sense of his greatness. To have achieved so much, to have lived such a life, to have been so fundamentally sane despite his affliction is astonishing. And there's something deeply moving about Boswell's concerned awareness for his friend's extraordinary suffering.
For all our advances in understanding of the psychological problems that so many people courageously deal with, I'm not sure we see these with greater clarity or insight than the mighty biographer and his mightier subject.