John Maynard Keynes once described Sir Isaac Newton, perhaps the greatest figure of the scientific revolution, as being ‘‘not the first of the age of reason’’ but ‘‘the last of the magicians.’’ Keynes was commenting, among other things, on Newton’s fascination with alchemy and the influence it may have had on his mathematical studies of gravitation and optics. This quip, no doubt originally deployed for its pithiness, raises broad questions of historical periodization.