John Ruskin; The Stones of Venice (II, Chap 6)
. . . no good work whatever can be perfect, and the demand for perfection is always a sign of a misunderstanding of the ends of art. This for two reasons, both based on everlasting laws. The first, that no great man ever stops working till he has reached his point of failure; that is to say, his mind is always far in advance of his powers of execution. . . . The second reason is, that imperfection is in some sort essential to all that we know of life. It is the sign of life in a mortal body, that is to say, of a state of progress and change.