First, have a definite, clear practical ideal; a goal, an objective. Second, have the necessary means to achieve your ends; wisdom, money, materials, and methods. Third, adjust all your means to that end.
The line has magnitude in one way, the plane in two ways, and the solid in three ways, and beyond these there is no other magnitude because the three are all.
If purpose, then, is inherent in art, so is it in Nature also. The best illustration is the case of a man being his own physician, for Nature is like that - agent and patient at once.
If then, as we say, good craftsmen look to the mean as they work, and if virtue, like nature, is more accurate and better than any form of art, it will follow that virtue has the quality of hitting the mean. I refer to moral virtue [not intellectual], for this is concerned with emotions and actions, in which one can have excess or deficiency or a due mean.
A tragedy is the imitation of an action that is serious and also, as having magnitude, complete in itself . . . with incidents arousing pity and fear, wherewith to accomplish its catharsis of such emotions.