No man pays double or twice for the same thing, forasmuch as nothing can be spent but once.

As Hobbes had centered on peace Petty chose prosperity. Further restrictions 'would do us twice as much harm as the losse of our said Trade' (p. 113)

What is striking about these passages is his intellectual rigour which put him far ahead of the mercantilist writers of earlier in the century.

He developed efficient methods to survey the land that was to be confiscated and given to Cromwell's soldiers. Sir William Petty FRS (26 May 1623 – 16 December 1687) was an English economist scientist and philosopher.