Petty, and, as was to be expected, he gives widely varying estimates of the same things[1]. It must be added that he is frequently inaccurate in his use of authorities[2] and careless in his calculations[3] and upon at least one occasion he is open to suspicion of sophisticating his figures[4].
Petty's economic writings thus exhibit both the strength and the weakness of his characteristic method. When his terms of number, weight and measure result from an actual enumeration they are generally of value, for he has a considerable capacity of segregating the really significant factors of an economic problem. But the difficulties in the way of enumeration were great, and in his eagerness for results he often resorted to calculations which were nothing more than guesses. When he stopped to think, he was well enough aware of their conjectural character. "I hope," he writes to Aubrey, "that no man takes what I say about the living and dyeing of men for a mathematical demonstration[5] ." But in the ardour of argument he was himself more than once mislead into fancying that his conclusions were accurate because their form was definite. His mistake is not without its modern analogies. Mathematical presentations of industrial facts, both symbolic and graphic, have by their definiteness, encouraged many an investigator in the false conceit that he now knew what he sought, whereas he had at most but a neat name for what he sought to know. Nevertheless the substitution of symbols for Petty's "terms of number" is an improvement in this, that calculations made in symbols must be consciously translated into the terms of actual life before any practical use—or misuse—can be made of them, whereas calculations in figures of number, weight and measure are already concrete and appear to tell something intelligible even to a common man. Had Petty calculated the advantages of his "perpetual settlement of Ireland with a natural improvement of England and Ireland by transplanting a million of people out of Ireland into England" in the form of curves and triangles, that astounding proposition might have passed for something highly scientific.
