Constraint

https://commons.wikimedia.org/wiki/File:Animaux_sauvages_-_ll._de_Jean_Matet_-_btv1b10573583s_(11_of_12).jpg

Art is limitation; the essence of every picture is the frame. If you draw a giraffe, you must draw him with a long neck. If, in your bold creative way, you hold yourself free to draw a giraffe with a short neck, you will really find that you are not free to draw a giraffe.

— G.K. Chesterton, Orthodoxy, 1908