This chapter discusses statistics as social science. The systematic study of social numbers in the spirit of natural philosophy was pioneered during the 1660s, and was known for about a century and a half as political arithmetic. Its purpose, when not confined to the calculation of insurance or annuity rates, was the promotion of sound, well-informed state policy. Political arithmetic was, according to William Petty, the application of Baconian principles to the art of government. Implicit in the use by political arithmeticians of social numbers was the belief that the wealth and strength of the state depended strongly on the number and character of its subjects. Political arithmetic was supplanted by statistics in France and Great Britain around the beginning of the nineteenth century. The shift in terminology was accompanied by a subtle mutation of concepts that can be seen as one of the most important in the history of statistical thinking.