Peter Quennell, Romantic England : writing and painting 1717-1851, Weidenfeld & Nicolson, 1970

Blake was as clever as a caddis fly in the use to which he put his borrowing ; but nothing could take his conviction that each of his angels and devils was the portrait of a supernatural sitter, who had appeared to him on one of his walk round London, or had crossed the threshold of his studio.