When we cover a piece of paper with doodles, when a child arranges colorful pebbles on his mud pie, or when we plant flowers in our gardens, one quality is common to all of these quite different activities, namely the enrichment of the outer world by the addition of perceptual elements.

Like the need for activity, it is a final, irreducible psychological fact – an urge in man not to be absorbed passively into his environment, but to impress on it traces of his existence beyond those of purposeful activity.