Marvin is described, both by Arthur and by himself, as "manically depressed". This is a contradiction in terms; mania is a state of excessive emotional high, just as depression is a state of excessive emotional low, so although they can both occur in the same person (it's called manic depression, cyclothymia, bipolar disorder or moodswing), there is no way they can possibly both occur in the same person at the same time.

(I know, I used to be cyclothymic.)