Anybody who has tried their hand at an extended piece of creative composition will understand what Emerson is going on about here:
Our moods do not believe in each other. To-day I am full of thoughts, and can write what I please. I see no reason why I should not have the same thought, the same power of expression, to-morrow. What I write, whilst I write it, seems the most natural thing in the world; but yesterday I saw a dreary vacuity in this direction in which now I see so much; and a month hence, I doubt not, I shall wonder who he was that wrote so many continuous pages. Alas for this infirm faith, this will not strenuous, this vast ebb of a vast flow! I am God in nature; I am a weed by the wall.
This is from his wonderful essay ‘Circles’. Click on the link above, and take a look. Well worth the fifteen minutes!