Those who are in love with practice without knowledge are like the sailor who gets into a ship without rudder or compass and who never can be certain whether he is going. Practice must always be founded on sound theory, and to this Perspective is the guide and the gateway; and without this nothing can be done well in the matter of drawing.

The Notebooks of Leonardo da Vinci (Richter, 1888) - I Prolegomena and General Introduction to the Book on Painting