To say to the painter that Nature is to be taken as she is, is to say to the player that he may sit on the piano.
When you have thoroughly learnt perspective, and have fixed in your memory all the various parts and forms of things, you should often amuse yourself when you take a walk for recreation, in watching and taking note of the attitudes and actions of men as they talk and dispute, or laugh or come to blows one with another, both their actions and those of the bystanders who either intervene or stand looking on at these things; noting these down with rapid strokes in this way, in a little pocket-book, which you ought always to carry with you. And let this be of tinted paper, so that it may not be rubbed out; but you should change the old for a new one, for these are not things to be rubbed out but preserved with the utmost diligence; for there is such an infinite number of forms and actions of things that the memory is incapable of preserving them, and therefore you should keep those (sketches) as your patterns and teachers.
Two men stop to talk together: I pencil them in detail, beginning at the head, for example; they separate and I have nothing but a fragment on my paper. Some children are sitting on the steps of a church; I begin, their mother calls them; my sketch-book becomes filled with tips of noses and locks of hair. I make a resolution not to go home without a whole figure, and I try for the first time to draw in mass, to draw rapidly, which is the only possible way of drawing, and which is to-day one of the chief faculties of our moderns. I put myself to draw in the winking of an eye the first group that presents itself; if it moves on I have at least put down the general character; if it stops, I can go on to the details. I do many such exercises, and have even gone so far as to cover the lining of my hat with lightning sketches of opera-ballets and opera scenery.
There is my model (the artist pointed to the crowd which thronged a market-place); art lives by studying nature, not by imitating any artist.
When you have clearly and distinctly learned in what good colouring consists, you cannot do better than have recourse to nature herself, who is always at hand, and in comparison of whose true splendour the best coloured pictures are but faint and feeble.