Can you tell me what the meaning of this quote is?

Is this specifically from "The book of the Duchess"?

I don't know from which novel this is from. It's by Ralph Aldo Emerson though.

I thought so. Try to link your question with a work that is related . In any case it is always good to consider the whole quote for better context, "With consistency a great soul has simply has nothing to do. He may as well concern himself with his shadow on the wall". Here Emerson is talking about the ability to try something new and change one's mind. Transcendentalists like Emerson were into the idea of discovery. Going with the status-quo, as many of us do, never leads to discovery or change. Doing and thinking the same things all the time is like studying a shadow on the wall. There is nothing but emptiness to be found in a stagnant mind. Doing new things and considering new ideas is a path to enlightenment. Hope this helps!