Fatio—generally so birdlike-quick in his answers—allowed some moments to pass. “Your question is a bit vague. Gold is important to Alchemists,” he allowed, “as comets are to astronomers. But there are some, of a vulgar turn of mind, who suppose that Alchemists are interested in gold only in the same sense as bankers are.”
“C’est juste. Though there is a troublesome banker, not far from here, who seems to value it in both the monetary and the Alchemical sense.” Leibniz, who until this point in the conversation had been the embodiment of good cheer, deflated as he was saying these words, as if he had been reminded of something very grave, and his eye strayed over to the outlandish red-leather book. This topic had had the same effect on his spirits as a handful of earth tossed into a fire. Again, Fatio allowed some moments to pass before he responded; for he was studying Leibniz carefully.
“I think I know who you mean,” Fatio said finally.
“It is most curious,” Leibniz said. “Perhaps you have heard some of the same stories concerning this as I have. The entire controversy, as I understand it, revolves around a belief that there is a particular sample of gold, whose precise whereabouts are unknown, but that possesses some properties that make it more valuable, to Alchemists, than ordinary gold. I would expect a banker to know better!”
“Do not make the error of believing that all gold is the same, Doctor.”
“I thought Natural Philosophy had proved at least that much.”
“Why, some would say it has proved the opposite!”
“Perhaps you have read something new in London or Paris that I have not seen yet?”
“Actually, Doctor, I was thinking of Isaac’s Principia.”
“I have read it,” Leibniz said drily, “and do not recollect seeing anything about gold.”
“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-à-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.