When you have well learnt perspective and have by heart the parts and forms of objects, you must go about, and constantly, as you go, observe, note and consider the circumstances and behaviour of men in talking, quarrelling or laughing or fighting together – the action of the men themselves and the actions of the bystanders, who separate them or who look on. And take a note of them with slight strokes, in a little book which you should always carry with you. And it should be of tinted paper, that it may not be rubbed out, but change the old when full for a new one; since these things should not be rubbed out but preserved with great care;