Here are some golden lines of his from Elements of Drawing:
BOLD, IN THE SENSE of being undaunted, yes; bold in the sense of being careless, confident, or exhibitory, --no, --no, and a thousand times no; for, even if you were not a beginner, it would be bad advice that made you bold. Mischief may easly be done quickly, but good and beautiful work is generally done slowly; you will find no boldness in the way a flower of a bird's wing is painted; and if nature is not bold at her work, do you think you ought to be at yours? So never mind what people say, but work with your pencil point very patiently. . . though there are all kinds of art there is one quality, and, I think, only one, in which all great and good art agrees; -- it is all delicate art. Coarse art is always bad art. . .you do not yet know how much tender thought, and subtle care, the great painters put into touches that at first look coarse. . .