Science fiction is a young man’s game. Many of the genre’s most dedicated readers, of course, are women, or men a decade or two removed from their prime. But no one, it seems, approaches the canon with greater reverence and zeal than the youthful and the easily moved.It’s not hard to understand why. Read a novel, and you’re invited to observe a variation on what is, fundamentally, your own life; the protagonists may be more eloquent, the setting more historically expansive, but there’s no mistaking the shared thrust of humanity, that subtle scent of hope and folly that you share with, say, every one of Tolstoy’s creations. A science-fiction novel, on the other hand, makes different demands—it lives and dies by the rules it has set up to govern its universe. Often, characters and plot are secondary to conceits and ideas. Young men read science-fiction novels for the same reason they join radical ideological movements: There’s no greater thrill than the promise of a new world, operating under new rules, in which you, slightly misjudged and deeply misunderstood, could soar to the furthest extent of your glory.A serious reader of science fiction is required to accept or reject not only the world of the novel but also the core concepts on which the world is based. Science-fiction writers are required to do the same thing; like seminary students, they spend their days parsing systems of knowledge that lie just ever so slightly beyond their reach. Which, perhaps, is why so many of them were atheists or otherwise had a problematic relationship with religion. Arthur C. Clarke, for example, insisted, upon his induction to the Royal Air Force, that his dog tag mark him as a pantheist. Isaac Asimov identified intermittently as an atheist and as a humanist, and he published his own historical-based analysis of the Bible. And Robert A. Heinlein, in his Time Enough for Love, underscored the difficulty of belief by having one of his characters state that to ponder the great mysteries of the universe, one would have to stand outside the universe, looking in. Which, of course, is exactly what we humans can’t do.Except for Olaf Stapledon.A British philosopher and author, a pacifist who spent World War I driving an ambulance on the front lines, an agnostic who produced some of his era’s most original and deeply felt works of religious thought, and maybe Baruch Spinoza’s most influential 20th-century disciple, Stapledon became famous in his time as the author of Star Maker, an astonishing—and astonishingly forgotten—science-fiction masterpiece. Virginia Woolf was a fan, as was Winston Churchill. Clarke was a disciple. And C.S. Lewis was so enraged by the work that he responded by writing not one book but three.What attracted such awe and ire is precisely what makes Star Maker so impossible to describe. The plot, if there is one, involves a British man who one night and for some unknown reason takes leave of our verdant planet and sails the cosmos, disembodied, to learn of its inhabitants and machinations. Much of the book consists of intricate descriptions of interstellar peoples; some, like the Other Men, are muddled by greed and pornography and omnipresent media and are deeply recognizable to us, while others are beautifully abstract.None, however, are entirely alien. Resisting invention for its own sake and shunning the sort of easy metaphor that relies on bits of imagination and scraps of scientific knowledge, Stapledon invested even his strangest creation with a sense of humanity, because he believed that the entire cosmic landscape shares the same matter and the same spirit. Early on in the novel, its human protagonist reaches this realization, and he is soon busy melding his mind with those of the creatures he encounters. “In time,” Stapledon wrote, “it became clear that we, individual inhabitants of a host of other worlds, were playing a small part in one of the great movements by which the cosmos was seeking to know itself, and even to see beyond itself.”Spinoza couldn’t have put it better himself. The Jewish philosopher was excommunicated in part for arguing that God was not “like man, consisting of a body and a mind, and subject to passions,” but rather an infinite substance. “Whatever is,” he wrote, “is in God, and nothing can be or be conceived without God.”When they finally meet the book’s eponymous Star Maker, Stapledon’s space travelers realize just how firmly they are in Spinoza’s debt. The philosopher isn’t mentioned by name, but his ideas bob just below the surface. Stapledon’s Star Maker, like Spinoza’s God, isn’t a singular creator who fashioned life out of nothingness with a single stroke of divine will but merely that entity from which all things flow, the cause of all existence—which means that we, too, are of him, minuscule parts of his divine intellect, eager to learn more.Such thought was heretical when Spinoza expressed it in the 17th century, and it was no less shocking coming from Stapledon in 1937. C.S. Lewis, a great admirer, nonetheless condemned Stapledon’s “desperately immoral outlook” and wrote his Space Trilogy to reaffirm his Christian belief. Lewis’ is a world of humans and demons, of strict codes and looming sin, the sort of heavily moralistic environment that he would later perfect in the Chronicles of Narnia. His turn to fantasy is no coincidence: While closely aligned with science fiction, fantasy is a far more elastic genre, one in which the great questions of this world are often set aside in favor of the luminous splendors of others.It is a rather precise testament to our spiritual moment that Lewis’ mythology is revered while Stapledon’s complex theology is left for the passionate few. Most of us would rather embrace some ready-made hagiography of boy wizards or charismatic lions than read through an account of an ordinary man striving to understand that which is infinitely greater than himself. But this struggle, this desire to know, this conviction that even when empirical evidence is scarce rational explanations are still within reach—this is the spirit that has propelled mankind forward for centuries, and it must continue to be ours: The key to human evolution, Stapledon would argue, is to cultivate an open mind.Liel Leibovitz is a senior writer for Tablet Magazine and a host of the Unorthodox podcast.