The sunlight is inconstant. The clouds come and go. The breeze moves the leaves of the tree outside the window. The angle of the sun above the horizon shifts as the earth spins ever farther past high summer. That distant fireball wanders southward, then west, as the morning turns to afternoon, and the afternoon fades into evening. Between the first sketched lines at quarter-past-nine and my morning snack at half-past ten, the shadows have already transformed the tomatoes. The color is different, the whole scene less sharp, more mellow, more muted. Still life is not still.
One of the unexpected and beautiful lessons of my first few weeks learning to be a working artist is that nature is not inclined to accommodate the punch-card and the office clock; it prefers that you time your work days with the sun and the clouds, and the life-cycles of the plants. The basil shifts color from deep green to bright-green-yellow over the course of a week of drawing. Tomatoes are perishable - nature to the rescue as you dither about which of the splendid array of shapes, colors, patterns, and sizes you've brought home from your farmers marketing to draw. (Eating is a rather enjoyable method of editing, it seems...) So you must, standing in the magical swirl of forces far bigger than yourself - forces that will not pause for you to draw their picture - take up your pencil and your artist's license, and represent only some small kernel of the truth of the world. How humbling, and how freeing.