The six-hundred page book Leonardo wrote to explain those ideas is the bridge that connects him to the present day. We may not have a detailed historical record of Leonardo the man, but we have his words and ideas. Just as we can come to understand great novelists through their books or accomplished composers through their music — particularly if we understand the circumstances in which they created — so too we can come to understand Leonardo of Pisa. We know what life was like at the time he lived. We can form a picture of the world in which Leonardo grew up and the influences that shaped his ideas. (In that we are helped by the survival to this day, largely unchanged, of many of the streets and buildings of thirteenth century Pisa.) And we know how numbers were used prior to the appearance of Liber abbaci, and how the book changed that usage forever.