What is your definition of number? The question is a challenging one because defining the abstract idea of number is extremely difficult. More than 2,500 years ago, the great number enthusiast Pythagoras described number as "the first principle, a thing which is undefined, incomprehensible, and having in itself all numbers." Even today, we still struggle with the notion of what numbers mean. Numbers neither came to us fully formed in nature nor did they spring fully formed from the human mind. Like other ideas, they have evolved slowly throughout human history. Both practical and abstract, they are important in our everyday world but remain mysterious in our imaginations.