Leonardo had studied the use of math for commercial purposes at his abacus school, but from Verrocchio he learned something more profound: the beauty of geometry.

Skill without imagination is barren. Leonardo knew how to marry observation and imagination, which made him history’s consummate innovator.

And now, the Mona Lisa (fig. 134). The discussion of Leonardo’s pièce de résistance could have come earlier in this book. He began working on it in 1503, when he returned to Florence after serving Cesare Borgia. But he had not finished it when he moved back to Milan in 1506. In fact, he carried it with him, and continued to work on it, throughout h