René Descartes's skull is small, jawless and looks as if it's been varnished. It sits in a display case in Paris's Musée de l'Homme on top of a faded edition of his Discours de la Méthode and next to three other skulls: one from a 100,000-year-old Homo sapiens, another from a 40,000-year-old Cro-Magnon man, and a third belonging to one of the earliest French farmers, dating back about 7,000 years. Truth be told, Descartes's isn't the most impressive of the four. I'd have thought it would be larger, to contain the brain of the genius whose philosophical method of doubt gave us the key to the Enlightenment and free thinking.
    On the skull's forehead, you can still make out Latin phrases and the signatures of previous owners, who probably had curiosity cabinets. These markings are all the more fascinating in that we now know-beyond a shadow of a doubt-that the history of this skull is extraordinary indeed. The story begins in 1666, 16 years after the exiled philosopher died in Stockholm. That was the year that Descartes's remains left Sweden in the baggage of a French diplomat, Hugues de Terlon, to be interred in Paris, at the Eglise Sainte-Geneviève. Then came the Revolution, and church tombs were dug up to save them from being sacked; Descartes's bones are believed to have been transferred to a site that is now l'Ecole des Beaux-Arts for safekeeping. Later, discussions arose concerning possible interment in the Pantheon, but the Eglise Saint-Germain-des-Prés was finally chosen to be the philosopher's last resting place. That was when they discovered that the skull was missing. It was tracked down in Sweden and began its own travels, almost getting swept away in a flood.
    You can read about all these skeletal peregrinations in an excellent book called Descartes' Bones by Russel Shorto (Doubleday, 2008). It's a riveting tale, part historical adventure and part philosophical digest, that takes the form of a detective story featuring a somewhat masculine Nordic queen, a very Cartesian yet very Catholic (I see a contradiction there, others say it's possible) French diplomat, a Swedish casino owner, poets, scholars, astronomers and physicists, revolutionaries, and even Louis XIV and a few Republicans. Descartes's skeleton and, more specifically, his skull, experienced so many adventures that they are the subject of a Cartesian-style study marked by skepticism and the search for truth.
    For more than 350 years, Shorto points out, Descartes's ideas took root, ushering in the dawn of scientific thinking and an imperious desire for democracy and reason-reason that defies faith, the Church and monarchies ruling by Divine Right. Yet the philosopher's remains were treated like relics of the saints or splinters of the True Cross. Like some sort of superstitious charm, the skull was coveted and traded. Through this paradoxical story, we relive some of France's most moving moments, from the first stirrings that would become the Enlightenment to the passions and the violence of the Revolution. We meet the young Alexandre Lenoir, a convert to the ideas of the Revolution and equality who nonetheless saved religious statues and tombs of great men from the destruction of the sans-culottes. He may even have saved Descartes's, although that's by no means certain. We also witness the impassioned debates at the Académie des Sciences where, in the 19th century, scholars still pitted faith against reason or tried to reconcile the two, as Descartes himself had done (although in his day, he was accused of atheism).
    Shorto goes on to describe how, true to their master's spirit, Cartesians sought to prove the authenticity of the little skull in the Musée de l'Homme through scientific means (a delicate task, given that it was the object of the debate about the duality of body and spirit). In 1913, physician, anatomy scholar and sculptor Professor Paul Richer used a portrait of Descartes by Frans Hals to sculpt a hollow bust, then made a cast of the skull, the idea being to show whether the internal and external parts would fit together like hand in glove. The result, now on view at Paris's Académie des Beaux-Arts, is rather strange and very morbid-a sort of classical bust whose face comes off to reveal its mystery: the very image of death as symbolized by a skull.
    Doubt, of course, is an integral part of the Cartesian approach. It is not at all certain, despite all efforts and Richer's talents, that the skull in the Musée de l'Homme really belonged to Descartes. Nor is it certain that the famous portrait of Descartes attributed to Frans Hals was really his work or even, says Shorto, that the bones in the tomb of the Eglise de Saint-Germain-des-Prés are really the philosopher's. I find this quest without a certain conclusion admirable, and I think (therefore I am) that this story is nonetheless useful, because it shows once again that reason is the path to knowledge, but that certainty is the monopoly of idiots.