From Volume 2 of Ruskin's great work.
"[...] No good work whatever can be perfect, and the demand for perfection is always a sign of the misunderstanding of the ends of art. This is 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, and the latter will now and then give way in trying to follow it; besides that, he will always give to the inferior portions of his work only such inferior attention as they require; and according to his greatness he becomes so accustomed to the feeling of dissatisfaction with the best he can do, that in moments of lassitude or anger with himself he will not care though the beholder be dissatisfied also. I believe there has only been one man who would not acknowledge this necessity, and strove always to reach perfection, Leonardo; the end of his vain effort being merely that he would take ten years to a picture and leave it unfinished. And therefore, if we are to have great men working at all, or less men doing their best, the work will be imperfect, however beautiful. Of human work, none but what is bad can be perfect, in its own bad way. [...] The foxglove blossom, - a third part bud, a third part past, a third part in full bloom, - is a type of the life of this world. And in all things that live there are certain irregularities and deficiencies which are not only signs of life, but sources of beauty. No human face is exactly the same in its lines on each side, no leaf perfect in its lobes, no branch in its symmetry. All admit irregularity as they imply change, and to banish imperfection is to destroy expression, to check exertion, to paralyze vitality. [...] Accept this then for a universal law, that neither architecture nor any other noble work of man can be good unless it be imperfect. [...] Our building must confess that we have not reached the perfection we can imagine, and cannot rest in the condition we have attained. If we pretend to have reached either perfection or satisfaction, we have degraded ourselves and our work. [...] It is that strange disquietude of the Gothic spirit that is its greatness; that restlessness of the dreaming mind, that wanders hither and thither among the niches, and flickers feverishly around the pinnacles, and frets and fades in labyrinthine knots and shadows along wall and roof, and yet is not satisfied, nor shall be satisfied. [...] The work of the Gothic heart is fretwork still."