Facts and history utterly contradict this view.

Most of the very great poets have been not only sane, but extremely business-like; and if Shakespeare ever really held horses, it was because he was much the safest man to hold them.
Imagination does not breed insanity.

Exactly what does breed insanity is reason.

Poets do not go mad; but chess-players do.

Mathematicians go mad, and cashiers; but creative artists very seldom.