"Montaigne was a French courtier who retired from political life in 1571 to sit in a castle tower and reflect on vanity and happiness, on liars and friendship. While he found comfort in this solitude, pain intruded on his contemplation from time to time, thanks to his kidney stones. One day, Montaigne transformed the stones into grist for an essay. "It is likely I inherited the gravel from my father," Montaigne guessed, "for he died sadly afflicted by a large stone in the bladder." Yet Montaigne had no idea how one could inherit a disease, as opposed to a crown or a farm. His father had been in perfect health when Montaigne was born, and remained so for another twenty-five years. Only in his late sixties did his kidney stones first appear, and they then tormented him for the last seven years of his life. "While he was still so remote from the disease, how could the light trifle of his substance out of which he built me convey so deep an impress?" Montaigne wondered. "Where could the propensity have been brooding all this while?" Simply musing in this way was a visionary act. No one in Montaigne's day thought of traits as being distinct things that could travel down through generations. People did not reproduce; they were engendered. Life unfolded as reliably as the rising of bread or the fermenting of wine. Montaigne's doctors did not picture a propensity lurking in parents and then being reproduced in their children. A trait could not disappear and be rediscovered, like a hidden letter. Doctors did sometimes observe certain diseases that were common in certain families. But they didn't think very much about why that was so. Many simply turned to the Bible for guidance, citing the passage telling of God "visiting the iniquity of the fathers upon the children unto the third and fourth generation."