Hamish McRae's The World in 2020 begins its discussion of population with this blunt sentence: "Of all the forces that will change the world over the next generation, demography is probably the most important." 1 agree. After all, men, women, and children determine the demand for things; men and women determine the size of the workforce; and if the supply of goods and services they produce and export is not adequate, people go hungry, lack medical services, and all too often perish too young. The rhythm of human life is such that those who are born now will, by and large, live through the middle of the next century. We owe them some things. However, as this chapter argues, the future is complicated by more than simply the rate of increase of the population. There are those who do not trace the beginning of modern economics to David Hume, Adam Smith, and their colleagues in the Scottish Enlightenment of the 18th century. They prefer the "Political Arithmeticians"— the statisticians—of the late 17th century, the greatest of whom was William Petty. Petty ranged widely over the field of economics including some wise and subtle reflections on the role of minorities in international trade. In 1695, Gregory King estimated the national accounts of England and Wales as of 1688. He used, essentially, a modern balance-sheet method, demonstrating the relationships between output and expenditure for five sectors of the economy. But it was John Gaunt as early as 1662 who cast the longest shadow on the future with his estimates of death rates in London based on the bills of mortality. His work is the beginning of modem demography. What stirred these late-17th-century inquiries? It was not a precocious academic interest in measuring population and national income; it was a sense that the nations of Europe were emerging from the feudal past and its internal struggles for power into an international arena of hostility and combat. In the following century, Britain and France, for example, were at war for more than 43 years.