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.
_Leonardo._