A Treatise on Painting/Chapter 156
Chap. CLVI.—On the Attitudes of the human Figure.
A painter is to be attentive to the motions and actions of men, occasioned by some sudden accident. He must observe them on the spot, take sketches, and not wait till he wants such expression, and then have it counterfeited for him; for instance, setting a model to weep when there is no cause; such an expression without a cause will be neither quick nor natural. But it will be of great use to have observed every action from nature, as it occurs, and then to have a model set in the same attitude to help the recollection, and find out something to the purpose, according to the subject in hand.