Petty's historical discussion of land valuation based on generational lifespans

Wherefore we must pitch upon some limited number, and that I apprehend to be the number of years, which I conceive one man of fifty years old, another of twenty eight, and another of seven years old, all being alive together may be thought to live; that is to say, of a Grandfather, Father and Childe; few men having reason to take care of more remote Posterity: for ifa man be a great Grandfather, he himself is so much nearer his end, so as there are but three in a continual line of descent usually coexisting together; and as some are Grandfathers at forty years, yet as many are not till above sixty, and sic de eteteris. 20. Wherefore | pitch the number of years purchase, that any Land is naturally worth, to be the ordinary extent of three such person their lives. Now in England we esteem three lives equal to one and twenty years, and consequently the value of Land, to be about the same number of years purchase. Possibly if they thought themselves mistaken. ...(as the observer on the Bills of Mortality thinks they are...) 21....Butin other Countreys Lands are worth nearer thirty years purchase, by reason of the better titles, more people, and perhaps truer opinion of the value and duration of three lives. 23. One the other hand, Lands are worth fewer years purchase (as in Ireland) ... by reason of the frequent rebellions. . .” The “other Countreys” could include France and especially Holland, then models of prosperity. Petty had made his fortune in Irish mortgages, and knew the years purchase there. But the argument is a puzzle. There is a focus on longevity and mortality, as if the generations are providing for old age. But Petty’s overlapping generations model cannot be much like Paul Samuelson’s of three centuries later, where a generation of productives leaves a nest egg for retirement. Samuelson’s productives are replenished exogenously, with children left to the imagination. Why would Petty have mentioned their ages? And retirement at age 50, as anorm, would have made no sense to Petty or his readers. The grandfather will stay in harness. Chapter 7 Petty’s Idea 2/3/16 15 HOUSE_OVERSIGHT_011074