“How can I get in this head?” Leibniz reports thinking over and over again. “How? How? How? How can I get in this head? How can I get in this head? How? How can I get inside this head?” The problem of getting inside another head, and seeing what that head was seeing (or not seeing) and what it was thinking (or not thinking), now struck Leibniz as a profoundly philosophical problem. Neither cradling it nor cracking it open would do it, for the barrier involved not only bone but also a thick layer of philosophy. The human skull consists, one might say, Leibniz wrote, of a quarter-inch-thick layer of bone and a quarter-inch-thick layer of philosophy. Of course the brain is also cushioned by various membranes and fluids. A skilled doctor can penetrate the skull with a drill, and he can cut through the membranes with a knife, and he can drain the cerebral fluids with a pump, but his instruments are utterly useless for penetrating that solid, condensed layer of philosophy. “Even the most state-of-the-art medical instrument wielded by the best doctor in Paris will simply bounce off the cerebral-philosophical membrane,” Leibniz wrote. That left language. “Never had I been so reliant on words to expose to me the innards of another head, and never had words seemed so unequal to the task,” Leibniz wrote, noting that the astronomer, who had never once stopped talking while Leibniz entertained these troubled thoughts, had returned to the topic of Kepler and bread dumplings, and Tycho and meat. “These words were supposed to illuminate for me this old man’s mind? Words like ‘Kepler’ and ‘dumplings’ and ‘four o’clock’ and ‘five o’clock’ and ‘Tycho’ and ‘venison’ and ‘five o’clock’ and ‘six o’clock’ and words like ‘Kepler’ and ‘goulash’ and ‘seven o’clock’ and ‘no astronomer’ and ‘Tycho’ and ‘goose’ and ‘eight o’clock’ and ‘no astronomer,’ and also, for the first time, words like ‘Galileo’ and ‘fettuccine’ and ‘nine o’clock’ and ‘no astronomer’?” From these words was he really supposed to be able to determine what the astronomer saw, and whether he saw, as well as whether and what the astronomer thought?

Leibniz angled his pocket watch toward the light of the half-melted candle. It was now ten past nine. Through the warped slat he could see a small slice of the bright blue sky.