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The one and twenty years could mean remaining life expectancy at age 50. But Petty could easily have spelled that out, or the implied 71 year terminus. He does spell out the ages of the three generations. Their average difference in age rounds to 21 years. Petty’s readers, like Smith’s and Ricardo’s after, would have taken it for granted that each generation provides for the next. “Few men having reason to take care of more remote posterity” would have registered in the context of that provision. “Posterity” usually meant and means descendants. His description, like mine, is incomplete. He may mean that life expectancy is also a factor in calculating the years purchase. If so, he apparently leaves that thought to be followed up later. There is also room to argue that the grandfather looks two generations ahead, so that the years purchase becomes 42 years. But that would give the usus fructus at 2.3%. All the rates Petty reports elsewhere in the tract are much higher. One generation length is what he seems to apply. My reading is that the grandfather provides for the grandson by passing all to the son. Petty’s overlapping generation insight has been one of his least noticed, just as with Mill’s on output growth preceding and explaining capital growth. I first read of Petty’s idea in a collection of Lionel Robbins’ lectures at London School of Economics delivered in 1979-1980, but published in 2000. I learned from these lectures that Gustav Cassel had published the same idea in his The Nature and Necessity of Interest in 1903. I hunted that down. Robbins misremembered in telling his students that Cassel had arrived at the idea independently. In fact Cassel and Robbins both quote the same excerpts from A Treatise of Taxes that I just did. Cassel inferred that interest rates cannot stably be less than 2% per year. Chapter 7 Petty’s Idea 2/3/16 16 HOUSE_OVERSIGHT_011075