and absurd-the creationist who wants to compress the history of life into the few thousand years of a literal biblical chronology, or the few serious members of the Flat Earth Society. But a "modern" truth, espoused out of time by a scholar in the distant past, fills us with awe, and may even seem close to miraculous.

A person consistently ahead of his time-a real-life Hank Morgan who could present a six-shooter to Julius Caesar, or explain the theory of natural selection to Saint Thomas Aquinas-can only evoke a metaphorical comparison with a spaceman from a more advanced universe, or a genuine angel from the realms of glory. In the entire history of science, no man seems so well qualified for such a designation as Leonardo da Vinci, who died in 1519, but filled his private notebooks with the principles of aeronautics, the mental invention of flying machines and submarines, and a correct explanation for the nature of fossils that professional science would not develop until the end of the eighteenth century. Did he have a private line across the centuries to Einstein, or even to God Himself?

I must confess that I share, with so many others, a lifelong fascination for this man. I was not a particularly intellectual child; I played stickball every afternoon and read little beyond comic books and school assignments. But Leonardo captured my imagination. I asked, at age ten or so, for a book about his life and work, probably the only intellectual gift that I ever overtly requested from my parents. As an undergraduate geology major, I bought the two-volume Dover paperback edition of Leonardo's notebooks (a reprint of the 1883 compilation by Jean Paul Richter) because I had read some of his observations on fossils in the Leicester Codex,^l and had been stunned not only by their accuracy, but also by their clear statement of paleoecological principles not clearly codified before our century, and still serving as a basis for modern studies.

Leonardo remains, in many ways, a frustrating and shadowy figure. He painted only about a dozen authenticated works, but these include two of the most famous images in our culture, the Mona Lisa (in the Louvre) and the Last Supper (a crumbling fresco in Milan). He published nothing in his lifetime, despite numerous and exuberant plans, though several thousand fascinating pages of manuscript have survived, probably representing only about a quarter of his total output. He did not hide his light under a bushel and was, in life, probably the most celebrated intellectual in Europe. Dukes and kings reveled in his conversation and his plans for war machines and irrigation projects. He served under the generous patronage of Europe's most powerful rulers, including Ludovico il Moro of Milan, the infamous Cesare Borgia, and King Francis I of France.

Leonardo's notebooks did not become generally known until the late eighteenth century, and were not published (and then only in fragmentary and occasional form) until the nineteenth century. Thus, he occupies the unique and peculiar role of a "private spaceman"-a thinker of preeminent originality, but whose unknown works exerted no influence at all upon the developing history of science (for nearly all his great insights had been rediscovered independently before his notebooks came to light).²

The overwhelmingly prevailing weight of public commentary about Leonardo

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1. What goes 'round, comes 'round-as Leonardo must have said somewhere. This Leicester Codex, one of Leonardo's most important notebooks, filled largely with commentary on the nature and use of water, first came to light in the 1690s, when Giuseppi Ghezzi found the document in a chest of manuscripts in Rome. In 1717, Thomas Coke, late Lord Leicester (hence the codex's name), purchased the notebook, which remained in his family until Armand Hammer bought it in 1980-and renamed it, in Trumpian fashion, Codex Hammer. With enormous fanfare (at enormous profit), Christie's auctioned this notebook on November 11, 1994, where America's Bill Gates outbid several European governments, and bought the manuscript for more money than I can count. Gates, to his credit, restored the original name, and has favored public exhibition of the document-including a show at the American Museum of Natural History in 1996, where I finally saw this icon of my dreams and admiration, and where I developed the ideas for this essay. The Leicester Codex is the only manuscript of Leonardo's now residing in America.