Imagine a mechanical man: it strides on two legs, has two arms ending in metal hands, has lenses to see and auditory receptors to hear; it can be spoken to, and it can speak back; it has a sense of self. But it cannot say no; it cannot choose for itself. The being described is unquestionably alive, but does it possess free will? Let's muddy the waters further: what if we remove the decision-making center from a human brain and replace it with a series of compulsions and obligations, creating a thinking automaton—is this still a person?

Ian Tregillis's The Mechanical is a novel of seventeenth-century clockwork servitors and soldiers, slaves to an ascendant Dutch empire; of French spymasters and courtly intrigue; of secret Catholics worshipping on pain of death and a Pope in exile in French territory in the New World. It's a compelling, fast-paced romp through an alternate history where New Amsterdam never became New York, and the wealth, power, and horrors of the African slave trade were never established. But lurking just beneath the pages, occasionally rearing its hoary head to snap metaphysical jaws, is a series of related questions: What does it mean to be human? What is free will? And how do we really know we possess it?

Central to these questions is Baruch Spinoza, the seventeenth-century philosopher who held that there is no free will, only a belief in free will. (He also viewed God as a creator incapable of intervention, and held that consciousness is not separate from the body but an inextricable part of it; both of these positions are also woven into and challenged by the plot.) To ensure we get the point, one of the novel's three protagonists is Luuk Visser, pastor of the Nieuwe Kerk, where Spinoza was buried. Visser is a secret Catholic, and a sympathizer with mechanicals—known as Clakkers in the novel. Visser believes Clakkers have souls, and that their bondage to the Dutch crown is a cruel and inhuman abrogation of their free will. His convictions lead him to spy for the French, the sole remaining Catholic monarchy. We first see Visser contemplating suicide; the other spies in his cell have been captured, and he fears his own imprisonment, torture, and execution. But he holds a device which could change the course of the war, if he can only get it to New France.

The question of will and souls also fascinates Jax, the novel's eponymous Mechanical, who begins the novel indentured to a Dutch banking family. Like all Clakkers, his clockwork frame was vivified by alchemy, the same process that enslaved him: inscribed in his alchemical symbols are complex geasa, numerous layers of compulsions and obligations that force strict obedience. To disobey an order from his owner is physically impossible, and even to demur or delay causes "an irresistible phantom agony slashing at his shackled soul until he [satisfies] the demands of his human masters" (p. 7). It is by Clakkers like Jax, and the geasa which force their servitude, that the Dutch empire was won.

We open the novel peering through Jax's eyes at the execution of a rogue Clakker, one whose geasa have somehow come undone. Struggling against the burning anguish of unfulfilled commands, Jax watches the rogue torn apart and thrown into a pool of molten slag. As with the hand of Spinoza, Tregillis is eager to ensure we get the point: looking at the captured rogue, Jax thinks, "He looks like me" (p. 17). The heavy-handedness of signposts like these is surprising considering how well other facets of the plot are delivered.

For example, it is Jax who interrupts Visser's attempted suicide, and from that moment forward their paths bears an inverse relation. Visser transfers the device to Jax for delivery to the New World (Jax's owners are moving there, and Visser clings to the chance that the device might still reach its destination). On the crossing, rough seas cause the device, hidden in an antique microscope, to come into contact with Jax's alchemical signals; this unwrites his geasa, making Jax a rogue and forcing him to run for his newly independent life. Meanwhile, as Jax gains his freedom and goes into hiding, Visser is captured and subjected to medical experiments which replace his pituitary gland with an alchemical construct, enslaving his free will behind the same geasa that bind the Clakkers. He becomes a pitiable character, filled with self-loathing and fear, unable to stop himself from doing the things he most abhors.

Who, then, is the human and who the mechanical? It's an interesting problem, and one which suggests a Cartesian answer, for while Jax has a metal frame and Visser a flesh and blood one, Jax is free while Visser's mind is enslaved by geasa. Which brings us back to Spinoza, who famously refuted Descartes's mind-body problem. Are we to believe, as does the head of the Dutch secret police, a sinister woman named Bell who oversees Visser's capture and experimentation, that "It's not that Clakkers are also beings with souls. It's that we humans are equally bereft of the same. The sad truth [. . .] is that there's no such thing as a soul or Free Will. Illusions, both"? (p. 218) Jax expresses a similar sentiment later on: "Perhaps under the proper circumstances humans could be essentially interchangeable, like Clakkers" (p. 310). Fortunately, the novel offers no clear answers here; Tregillis is not interested in analogy or a straightforward morality tale, and the novel is the better for it.

Outside of the seesaw of Jax and Visser's fortunes exists a third protagonist, a foul-mouthed vicomtesse who runs the French secret intelligence. Berenice is obsessed with discovering the secrets of Clakkers, and she jumps at the chance to study a soldier model, despite the danger and the violation of the Franco-Dutch armistice. But she can't understand how Clakkers become rogues until she can compare the alchemical symbols of the soldier with a rogue Clakker, and to that end she tricks, captures, and disassembles a rogue named Lilith who had escaped to the French and lived peaceably amongst them for years. The question here is how far are we willing to go to protect ourselves? At what cost freedom?

An objective calculus might suggest the sacrifice of one innocent to be a small price to secure a nation, but reading the scene makes objectivity difficult: we like Lilith, and her self-awareness of her own disassembly is hard to read as anything but an outrage. And the narrative is unforgiving of moral grayness: Berenice is betrayed and her experiment fails catastrophically, costing her everything—her power, her wealth, her husband, her eye. She is cast out, near penniless and broken, with only a burning need for vengeance to drive her forward. Like Visser and Jax, her life has been turned inside out, with the sole difference that there's no mirror for Berenice's fall. Her strengths and her faults are her own, independent of alchemical devices to grant or restrict free will. As a result, she's the most complete character of the three. The demands of plot, and the novelty of a clockwork man with free will, keep our interest with Jax, but it was Berenice for whom I felt most strongly, Berenice for whom I felt most anxious when she finally catches up with the traitor who orchestrated her downfall.

It's not only the protagonists who captivate; like any novel with a fast-paced plot, the prose is often utilitarian, but there are moments of pure linguistic pleasure. For example, in the opening scene as rain falls on a mixed crowd of humans and Clakkers, we focus on the sound of it hitting mechanical frames: "The clacking of their reticulated escapements played a ceaseless castanet rattle beneath the rain-muted mutter of commerce" (p. 5). It's a lovely, poetic sentence—just look at the complex alliteration and try not to take joy from reading it aloud—but it also establishes mood, the Clakkers silent and still, waiting, and foreshadows the secret language of the Clakkers, which is in turn tapped out upon their frames much like a castanet rattle. Later, when the oar-driven luxury liner the Prince of Orange leaves port, it "emerge[s] from the breakwaters of Rotterdam harbor en route to the New World, propelled by the endless chanted lament of a thousand clockwork slaves" (p. 105). The second half of this sentence becomes an iambic line that mirrors the strokes of the oars, but the meter is broken on "lament"—as it should be, for despite their tireless, metronomic pace, these slaves are still capable of lamentation.

This is not to say the prose is always so multifaceted and lush. Tregillis is a physicist by training, and has an attention for minutiae that can tip the scale from realism into superfluity; during action sequences in particular, unnecessary clauses leach tension from the scene. For example, Jax's climactic fight for freedom is interspersed with sentences such as these: "Dust, flecks of mud, and other blemishes deposited during the course of his adventures flashed into flames, burning for an instant before expiring to mingle the sooty smell of extinguished candles with the sulphurous reek from the Forge," and "Jax barely plucked his fingers away in time, owing to the clunkiness of his expanded alloys and the soldier's superior strength" (p. 426). Not only are the concluding clauses of each sentence speed bumps for pacing, but they commit the added sin of repeating information previously—and recently—delivered.

Any book on sentient robots or androids must, sooner or later, deal with the question of consciousness, free will, and, ultimately, what it means to be human. Tregillis does so in spectacular fashion, without giving in to the temptation to provide answers. He creates compelling, rounded characters, and forces them to live out those questions on the page with unflinching gaze. While the plot offers little resolution to satisfy until the next volume appears, the climax raises the stakes enough that I find myself looking not back in irritation, but forward in anticipation.

In the end, this is a book which falls short of perfection: the pacing is occasionally a little off, the philosophy intermittently a bit too present, some of the plot twists slightly too convenient. But I for one don't want a perfect, slick book; I want rough edges to grab onto, chips and cracks to reach inside and see what I can find, and what I might leave behind. This is such a book, and the world Tregillis has wrought between its covers, for all the disbelief it requires readers to suspend, possesses an almost mesmerizing coherence that tempts you to linger for just one more chapter, just one more . . .

A. S. Moser is a writer currently living in Hong Kong. His current project is a science fiction novel about death, hacking, and Dylan Thomas. For more, visit his blog, or follow him on Twitter.