The imposition of the world's first modern income tax in 1799 prompted a revival of interest in national accounting. This article examines the extent to which William Pitt the Younger, who proposed the new tax, modelled his estimates of national wealth on those produced a century earlier by the pioneers in this field, Sir William Petty, Charles Davenant, and Gregory King. In addition, the calculations of Benjamin Bell and Henry Beeke, two of Pitt's contemporaries, are analysed in detail to highlight the fragility of these contemporary estimates of national income. This analysis has important implications for economic historians who have used this material to try to establish the structure and growth of national output. National accountants during the long eighteenth century were not, for the most part, concerned with structural change. Rather, their descriptions of economic structure should be understood as reflecting a particular set of a priori claims about what they deemed to be the proper mode and distribution of taxation.