“Doctor Leibniz mentioned to me long ago that there are two sorts of intellectual labyrinths into which all thinking people are sooner or later drawn,” said Caroline. “One is the composition of the continuum, which is to say, what is matter made of, what’s the nature of space, et cetera. The other is the problem of free will: Do we have a choice in what we do? Which is like saying, do we have souls?”

“I’ll agree with Baron von Leibniz at least to this point: these are interesting questions, and so many spend so much time thinking on them that the similitude of a labyrinth is well taken.”

Daniel reminded them, “The Princess has requested that this discussion be productive of a better System of the World. I put it to you that the latter question-free will, and the spirit-is, as far as that goes, the more important. Myself, I am comfortable with the notion that we are Machines made of Meat, that there’s no more free will in us than there is in a cuckoo-clock, and that the spirit, soul, or whatever you want to call it, is a f?ry-tale. Many who study Natural Philosophy will arrive at the same conclusion, unless the two of you find a way of convincing them otherwise. Her royal highness seems to be of the view that such beliefs, if they should be imbued into the new System that her House is erecting, shall lead to the realization of her nightmare. So, if I am to be Simplicio in this dialogue, pray explain how it is that there may be such a thing as free will, and a spirit that may do as it pleases, unbound by the Mathematick laws of our Mechanical Philosophy.”

“Well, if you put it that way, it’s an old problem,” said Leibniz. “Descartes saw straight away that Mechanical Philosophy might spell trouble for free will, in that it led to a new sort of predestinationism-not rooted in theology, like that of the Calvinists, but rather growing out of the simple fact that matter obeys predictable laws.”

“Yes,” said Daniel, “and then he got it all wrong, by putting the soul in the pineal gland.”

“I’d rather say he got it wrong before then, by dividing the universe into matter, and cogitation,” Leibniz said.

“And I’d say he got it wrong even before then, by supposing that there was a problem,” said Newton. “There’s nothing wrong in recognizing that part of the universe is a passive mechanism, and part of it is active and thinking. But Monsieur Descartes, seeing what was done to Galileo by the Papists, was in such terror of the Inquisition that his resolve failed.”

“Very well, in any case we agree that Descartes perceived a problem, and came up with a wrong answer,” said Daniel. “Does either of you have a better one to offer up? Sir Isaac, it sounds as if you deny the very existence of any such problem.”

“You may read Principia Mathematica without finding discourse of souls, spirits, cogitation, or what-have-you,” said Isaac. “It is about planets, forces, gravity, and geometry. I do not address, and certainly do not pretend to solve, the riddles that so confounded Monsieur Descartes. Why should we attempt to frame hypotheses about such matters?”

“Because if you do not, Sir Isaac, others, of less brilliance, will; and they will frame the wrong ones,” Caroline said.

Newton bristled. “My work on gravity and opticks has brought me a kind of fame, which is a thing I never sought, nor wanted. It has done me nothing good, and much bad-as now, when I am expected to utter profundities on topics far afield from what I have chosen to study.”

“So says the public Sir Isaac Newton,” said Daniel, “Author of Principia Mathematica, and Master of the Mint. But this is a private gathering, which might benefit from the participation of the private Sir Isaac: the author of the Praxis.”

“Praxis has not been published,” Isaac pointed out, “and not because I have deemed it somehow private but because ’tis yet unfinished, and so not fit to talk of.”

“What is Praxis?” Caroline inquired.

“What Principia Mathematica was to Mechanical Philosophy, Praxis would be to Alchemy,” said Isaac.

“A laconic answer! May we hear more?”

“If I may say so, highness,” said Daniel, “Sir Isaac learned early that anything he openly professed was liable to come under attack, to his great aggravation and embarrassment, and so became chary of professing anything until he had got it perfect, and made it impervious. Praxis is not ready yet.”

“Then it seems I shall not have any satisfaction whatsoever!” said Caroline, a bit poutingly.

“Which is entirely my fault, for having mentioned Praxis,” Daniel hastened to say. “But I had a reason for doing it, which was to say that, though the public Sir Isaac might profess not to see the problem that so captured the attention of Descartes, I believe that the private Sir Isaac has been working on just that problem.”

“As I state quite plainly in Principia Mathematica,” said Isaac, in a bit of a high clarion self-righteous tone, “it is not my intention, in that work, to consider the causes and seats of Force. That gravity exists, and acts at a distance, is taken as a given. Why and how it does so are not considered. I would not be human if I did not have some curiosity as to what gravity was, and how it works; and even if ’twere otherwise, Baron von Leibniz and his Continental supporters would never allow me a moment’s peace on the matter. So, yes! I would understand Force. I have toiled at it. The ignorant have styled my toils Alchemy.”

At this Daniel threw him an irritated look, which Isaac, to his credit, did not fail to notice. “C’est juste!” Isaac said. “It’s not wrong to call this work Alchemy, but that word, so laden with the baggage of centuries, doesn’t do justice to it.”

“May I ask a question about your research in this area-however you choose to denote it?” Leibniz asked.

“Provided it contains no hidden barbs or spryngs,” Isaac allowed.

Leibniz now achieved the difficult feat of rolling his eyes, heaving a great sigh of exasperation, and voicing his question all at the same time. “If I understand what ‘force’ means, in your metaphysicks-”

“Which is the only coherent definition of ‘force’ that I know of!” Newton slipped in, glancing at the Princess.

Leibniz, with some visible straining, affected a saintly mien during this. “It appears to mean some invisible influence, acting across what you think of as the vacuum of space at infinite speed, which causes objects to accelerate-even though nothing seems to be touching them.”

“Setting aside your strangely hedged and qualified way of talking about ‘vacuum’ and ‘space,’ that is a reasonable description of gravitational force,” Newton allowed.

“Now in your metaphysicks-which I concede happens to be that used by just about everyone-there is this thing called space, which is mostly empty, but has lumpy bits here and there, called bodies; some big heavy spherical ones which we call planets, but also any amount of clutter, such as this poker, yonder candelabrum, the rug, and these bipedal animated bodies answering to the names Daniel Waterhouse, Princess Wilhelmina Caroline of Brandenburg-Ansbach, et cetera?”

“That much is so obvious that some of us are amazed to hear a learned man waste breath pointing it out,” said Newton.

“Some of those bodies answer only to the deterministic laws of the mechanical philosophy,” said Leibniz, “such as the globe, which rolled into the fireplace because her royal highness gave it a shove. But the bodies denominated Daniel Waterhouse, et cetera, are somehow different. True, they are subject to the same forces as the globe-our friend Daniel plainly feels Gravity’s pull, or else he would float away! But such bodies act in complicated ways not explainable by the laws set forth in your Principia Mathematica. When Dr. Waterhouse sits down to write an essay, let us say about the Latitudinarian philosophy espoused by him and the late Mr. Locke, we may observe his quill maneuvering all over the page in the most complicated paths imaginable. Here are none of the conic sections of the Principia! No equation can predict the trajectory of Daniel’s nib over the page, for it results from innumerable and unfathomable minute contractions of the small muscles of his fingers and his hand. If we dissect a man’s hand, we find that these muscles are governed by nerves, which may easily enough be traced back to the brain, as rivers come from springs in the mountains. Remove the brain, or sever its connexions to the hand, and lo, that limb becomes as simple as yonder globe; that is, we may predict its future movements from the Principia, and plot them in Conics. And so it is evident that, to the Force of Gravity-which acts on everything-are superadded other forces, observable only in animals,* and productive of infinitely more complicated and interesting movements.”