“And yet it is clear enough that two planets of equal size and composition will describe different trajectories through the heavens, depending on their distances from the sun.”
“Of course-that is necessarily true, by the inverse-square law.”
“Since the two planets themselves are equal in every way, how can this difference in their trajectories be accounted for, unless you enlarge your scope of observations to include the difference in their situations vis-a-vis the sun?”
“Monsieur Fatio, a cornerstone of my philosophy is the identity of indiscernibles. Simply put, if A cannot be discerned from B, then A and B are the same object. In the situation you have described, the two planets are indiscernible from each other, which means that they ought to be identical. This includes having identical trajectories. Since they are obviously not identical, in that their trajectories differ, it follows that they must in some way be discernible from each other. Newton discerns them by assigning them differing positions in space, and then presuming that space is somehow pervaded by a mysterious presence that accounts for the inverse-square force. That is, he discerns one from the other by appealing to a sort of mysterious external quality of space…”
“You sound like Huygens!” Fatio snapped, suddenly annoyed. “I might as well have stayed in the Hague.”
“I am sorry if the tendency of me and Huygens to agree causes you grief.”
“You may agree with each other all you like. But why will you not agree with Isaac? Can you not perceive the magnificence of what he has achieved?”
“Any sentient man can perceive that,” Leibniz returned. “Almost all will be so blinded by its brilliance that they will be unable to perceive its flaws. There are only a few of us who can do that.”
“It is very easy to carp.”
“Actually it is rather difficult, in that it leads to discussions such as this one.”
“Unless you can propose an alternative theory that mends these supposed flaws, I believe you should temper your criticisms of the Principia.”
“I am still developing my theory, Monsieur Fatio, and it may be a long time before it is capable of making testable predictions.”
“What conceivable theory could explain the discernibility of those two planets, without making reference to their positions in absolute space?”
THIS LED TO AN INTERLUDE in the snow outside. Doctor Leibniz packed two handfuls of snow together between his hands, watched warily by Fatio. “Don’t worry, Monsieur Fatio, I’m not going to throw it at you. If you would be so helpful as to make two more, about the size of melons, as like to each other as possible.”
Fatio was not quick to warm to such a task, but eventually he squatted down and began to roll a pair of balls, stopping every couple of paces to pound away the rough edges.
“They are as close to indiscernible as I can make them under these conditions-which is to say, in twilight with frozen hands,” shouted Fatio towards Leibniz, who was a stone’s throw off, wrestling with a snowball that weighed more than he did. When no response came back, he muttered, “I shall go in and warm my hands if that is acceptable.”
But by the time Nicolas Fatio de Duillier had got back to Leibniz’s office, his hands were warm enough to do a few things. He took another look at the papers stuck into the Chinese book. The letter from Eliza was inordinately long, and appeared to consist entirely of gaseous chatter about what everyone was wearing. Yet on top of it was the other document, addressed to the Doctor but written in the Doctor’s hand. A mystery. Perhaps the book was a clue? It was called I Ching. Fatio had seen it once before, in the library of Gresham’s College, where Daniel Waterhouse had fallen asleep over it. The sheaf of papers had been used to mark a particular chapter entitled: 54. Kuei Mei: The Marrying Maiden. The chapter itself was a bucket of claptrap and mystickal gibberish.
He put it back where he’d found it, and went over to the shed’s single tiny window. Leibniz now had his back pressed against an immense snowball and was trying to topple it over by thrusting with both legs. Fatio strolled once around the room, pausing to riffle through any prominent stacks of papers that presented themselves to his big pale eyes. Of which there were several: letters from Huygens, from Arnauld, from the Bernoullis, the late Spinoza, Daniel Waterhouse, and everyone else in Christendom who had a flicker of sense. But one of the larger stacks consisted of letters from Eliza. Fatio reached into the middle, grabbed half a dozen leaves between his thumb and index finger, and snapped them out. He folded them and stuffed them into his breast pocket. Then he ventured back outside.
“Are your hands warm, Monsieur Fatio?”
“Exceeding warm, Doctor Leibniz.”
