Necessity, the third and concluding volume of Jo Walton’s Thessaly trilogy, is not a book that can be read on its own. You really do want to read the first two volumes, The Just City and The Philosopher Kings, first. As I pointed out in my review of those two books in 2011, this series is philosophical science fiction in the most literal sense. It’s the story of Greek gods creating a real-world city based on Plato’s Republic as a thought experiment, designed to explore concepts of consent, volition and equal significance. Told from the perspective of one of the gods, Apollo, who incarnates himself as Pytheas to live in the Just City, as well as several other humans brought to or raised there, it’s a story that combines a high-concept discussion of ethics — the purpose of the Just City is philosophy, and the characters talk about it endlessly — with strongly drawn characters.
Third books in a trilogy, or later books in a series, don’t get the same amount of critical attention as first books, even though many books in the genre appear as part of a series. Reviewing later books is often a questionable proposition for critics: The audience is limited to those who’ve read the previous books; if you have to read other books before turning to this one, this review isn’t of much use to you, especially if any discussion of book three unavoidably spoils books one and two. Much easier for the reviewer to select another title.
That said, there can be value in examining how a single volume fits into the larger narrative of the series, particularly in a field where those larger narratives are increasingly the rule. One can evaluate whether the series is concluded successfully — does the author stick the landing? Done well, a good ending can be transcendent, making the series more than the sum of its parts. And in more didactic works, the final volume can be the summation of an argument built over the course of the trilogy.
Necessity continues and extends the philosophical arguments begun in The Just City, if only a little bit. “Plato says that it is necessary for art to make an argument that it is beneficial to the soul as well as enticing to it,” says Crocus the Worker, who is (finally!) a viewpoint character this time around. The motif of equal significance and volition — the concepts Apollo began exploring at the outset of the series — recurs in each volume in the Thessaly trilogy; each book has its own particular focus. The Just City was about volition (i.e., consent), and explored questions of compulsion and slavery. The Philosopher Kings was about posterity — having your work endure and have meaning. Necessity explores the remaining part of Apollo’s concept: equal significance. If intelligent machines like Crocus have volition, do they also have souls? People are sorted into Golds, Silvers, Bronzes and Irons, with Golds as philosophers. Are Golds the only ones capable of philosophy? Do Irons and Silvers have the same significance as Golds?
The Philosopher Kings ended in an actual deus ex machina, when Zeus transported the whole enterprise from the pre-explosion island of Thera, thousands of years in the past, to a distant planet centuries in the future. Necessity picks up the story forty years later. The cities are connected by electric railways, sustain themselves by fishing the cold oceans of their new planet, which they’ve named Plato, and are in contact with two alien civilizations, notably the Saeli, some of whom have joined them on Plato (the planet) and studied Plato (the philosopher).
As the book opens, a number of things have suddenly happened. A human spaceship has appeared in orbit, and the inhabitants of Plato must prepare themselves for reestablishing contact with humanity. Pytheas, Apollo’s human incarnation, finally dies of old age; as he resumes his godhood, Apollo realizes that Athene has vanished from reality, going beyond time to explore the origins and fate of the universe. The search for Athene involves recovering the pieces of an explanation she left behind, a task undertaken by Pytheas/Apollo, the god Hermes, and Marsilia, a descendant of Simmea. They rely on Necessity (basically, a rule against temporal paradoxes and causality violations) to keep them safe as they go after her.
It’s something of a red herring, as is the arrival of the space humans; Necessity is not really driven by plot concerns. It does close out the trilogy by explaining mysteries that the first two books left explained (for example, Sokrates is back — we find out what happened to him after Athene turned him into a gadfly after The Just City) and it does tie up loose ends. Not that every mystery required explaining — one gets the sense of a complex equation being balanced at last. But mostly it’s another iteration of what Jo Walton does best: closely observed relationships between deftly drawn and sympathetic characters, who explore the philosophical questions both directly and indirectly, through both word and deed.
“I’ve been working on equal significance and volition for a long time now,” says Apollo at one point. “I think I understand something about them.” After three books, he could just as easily be speaking for the author — and now for the reader, too.