This quote is by Frank Vincent Dumond, who taught painting at the Art Students League in New York for nearly 60 years, and it should be the motto of every realist painter who paints from life. He taught my teacher, Frank Mason, how to mass, and Frank Mason taught me. I now teach it to my students. Massing is the underlying structure that every good painting should have to make it read as an illusion of light, form, and depth. Massing is learning to simplify and let go of the outline, and instead, think in terms of bulk, weight and advancing or receding in space.
I have this quote taped to the top of my easel as a reminder. Every painting is problem solving, and learning to solve your masses is key.