Butterworth's pioneering research into the behavior and genetics of mathematical ability has led him to discover that we all possess a fundamental number sense, which he calls "numerosity." We all know that some of us are good at math and some of us are not. But, as Butterworth shows, the reason a person falters at math is usually not because of the wrong gene or "engine part" in the left parietal lobe, but because he or she has not fully developed the sense we are all born with. Butterworth argues that counting is so basic a facet of our biology that, with practice, most people could become mathematical prodigies. The implications of Butterworth's advances in fundamental concepts of mathematical thinking are profound - for our understanding of how our minds work, how we can lead our children to a deeper understanding of mathematics, and even how formal education could be better structured on the basis of what counting really is.