Isaac Newton's two categories of chronological argument, by reasoning and by conjecture, are the two poles around which belief about the age of the world and the chronology of its history have always turned. But one man's conjecture is another man's reason, and Newton's contemporaries often seem to have had as much difficulty in deciding whether a particular chronological or chiliastical argument was conjectural, literal, cabalistical, revelational or rational, as they had in first formulating it. Borrowing from these categories, however, I might say that I am to consider arguments about the age of the world which were "rational" in the sense that Newton's chronology was thought to be so.¹ In other words, I shall try to bring together a group of specifically astronomical arguments developed out of premisses which were for the most part conjectural, if not positively nonsensical.