Apelles was in the habit of exhibiting his pictures to the passers-by, while he heard their comments without being seen. One day a shoemaker criticised the shoes in a certain picture, and found next day that they had been repainted. Proud of his success as a critic, he began to find fault with the thigh of the figure; when Apelles cried out from behind the canvas, Shoemaker, stick to your last.PLINY, H. N. 35. Told by Lucian of Phidias.
The success of Apelles was due to his constant practice, so that he allowed no day to pass without drawing at least a line, which Pliny formulated into a rule, No day without its line (Nulla dies sine linea).Ibid.