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Let no day pass over you without a line. Observe as you walk the streets, the airs of heads; the natural posturs and expressions; which are always the most free the less they seem to be observ'd. Be ready to put into your table-book (which you must always carry about you) whatsoever you judge worthy of it; whether it be upon the earth, or in the air, or upon the waters, while the species of them is yet fresh in your imagination.

Charles-Alfons du Fresnoy, De Arte Graphica, 1669