Myths are funny. Unlike histories, they are symbolic narratives; they deal with spiritual rather than fact-based truths. They serve as foundations for beliefs, illustrating how things came to be and who was involved, but they’re often sketchy about when or why. There’s a brief scene from Neil Gaiman’s new book Norse Mythology that does a remarkable job of capturing just this: the wonderfully nebulous sense of being in illo tempore—the hazy “at that time” of the mythic past. It begins, as many creation myths do, with “an empty place waiting to be filled with life,” but in this instance some life already exists. There’s Ymir, whose enormous body produces all giants and, eventually, the earth, skies, and seas. There’s Audhumla, the celestial cow, who licks the first gods out of blocks of ice. And there are three brothers—the gods Ve, Vili, and Odin—who must devise a way out of this timeless nowhere:

Ve and Vili and Odin looked at each other and spoke of what was needful to do, there in the void of Ginnungagap. They spoke of the universe, and of life, and of the future.

Odin and Ve and Vili killed the giant Ymir. It had to be done. There was no other way to make the worlds. This was the beginning of all things, the death that made all life possible.

It’s a passage that gives us a fascinating glimpse into an Old Norse worldview: It suggests that these early medieval people believed gods were many and formidable. That carefully chosen words could be as powerful as deeds. And that sacrifices must be made to secure a good future. In writing it, Gaiman has provided an enchanting contemporary interpretation of the Viking ethos. Like the Vikings themselves, his characters value ingenuity as much as physical prowess, since both help to build a memorable reputation. To paraphrase a sentiment attributed to Odin in the Viking Age poem Hávamál: Although everyone and everything eventually dies—giants and gods and brave warriors included—tales about praiseworthy folk will outlast them all. Myths about these impressive beings survive, then, because they captivate audiences; they survive because they’re continuously shared. And because they are shared, they change.

It’s perhaps less the notion of sharing than the possible changing of these Old Norse stories that sparked a mini-controversy last fall, when Gaiman announced his new book’s publication date. In less than three days, Gaiman’s Facebook post attracted more than 20,000 shares, 50,000 likes, and more than 3,200 comments. Reactions were polarized: On one side, throngs of fans were eager for the author’s recreation of these tales; on the other, a smaller, but no less vocal, group of self-proclaimed pagans seemed to dread his inevitable misunderstanding of their religious beliefs. At the time, none of these commenters had read Gaiman’s book.

Both the hype and the backlash seemed to stem less from the book itself, than from the way it had been promoted. Last June, The New York Times called Norse Mythology “an almost novelistic retelling of famous myths about the gods of Asgard,” while in July the book-discovery platform Bookstr asked, “Is Gaiman writing the definitive book on the gods?” This sort of marketing at once fed fans’ hopes that the author would produce another bestseller like American Gods (the 2001 Hugo and Nebula Award–winning novel currently being developed as a TV series for the Starz network), parts of which are inspired by, but nevertheless vastly different from, Norse stories. The implication that Gaiman has altered, revised, remade, or somehow improved the source material also incited a small-scale cultural appropriation debate in the comments to his post.

Gaiman is, it seems, fully aware of these discussions. In his introduction to Norse Mythology, he explains, “I’ve tried my best to retell these myths and stories as accurately as I can, and as interestingly as I can.” These are noble, sensitive aims—but the first of them, in the case of Norse myth, is ultimately futile. Each author tackling this material is like Ratatoskr, the squirrel running up and down the world tree Yggdrasil’s trunk, carrying messages from the dragon curled under its roots up to the eagle perched in its branches. Who knows what he’s forgotten on the long trip to the top? Since there’s no real “original” with which to make comparisons, it’s impossible to know precisely what a Norse tale sounded like in the first place.

Vikings weren’t known for their writing. That isn’t to say they didn’t have a way with words: They were skalds, storytellers, lawspeakers, singers. And many of the runes carved in the Viking Age (c.793-1066) survive today. However, these runic inscriptions are brief; wood, bone, and stone aren’t conducive to detailed narratives. The stories recognized today as pagan Norse myths were written down—and possibly reinvented—in more extended prose form by outsiders and Christians.

Tacitus, a Roman historian, wrote about Germanic peoples and their rituals centuries before they migrated to the British Isles. Ibn Fadlan, an Arab diplomat traveling the Volga trade route in the 10th century, described the funeral practices (ship burial and slave sacrifice among them) of the Rūs, a group of people believed to be Swedish Vikings angling to control eastern trade routes. Saxo Grammaticus, a Dane writing in Latin in the late 11th century, brought the Norse gods down to earth, downplaying their divine qualities and also situating their kingdom in Byzantium instead of in heavenly Asgard. Adam of Bremen, a German monk writing around the same time, shared stories about pagan worship at the temple in Uppsala, Sweden, one of early medieval Scandinavia’s most sacred sites. (Told second-hand based on an informant’s account, Adam’s frequently referenced work includes vague details about the blót ceremony held there every ninth year, at which nine specimens of every creature—including humans—were said to have been sacrificed to the gods.)

The vast majority of what is now known about Norse mythology, however, survives thanks to Snorri Sturluson, an ambitious and powerful chieftain, lawyer, politician, poet, and saga writer who lived in Iceland from 1179 to 1241. These dates are significant: They tell us that Snorri was recording these narratives roughly 200 years after the Christian conversion in Norway, Denmark, and Iceland. They also, significantly, tell us that “original” and definitively pagan narratives about the Norse pantheon do not actually exist.

This claim needs a bit of qualifying. Scholars mostly agree that the myths Gaiman has retold—the same ones found in Snorri’s Prose Edda—were inspired by earlier pagan narratives. In fact, several stanzas of pre-Christian poems are preserved in Snorri’s work. Other snippets of pagan poetry also appear in 13th and 14th century Icelandic sagas, truly novelistic accounts like Grettis saga and Egils saga (the latter also possibly written by Snorri). Yet by the time Snorri was composing his versions of the Norse myths, his worldview was solidly a Christian one.

Even if, for the sake of argument, it’s accepted that Snorri was channeling his pagan ancestors at the time of writing, that they were completely accurate 13th-century reports on pre-Christian mythology, we’d still be none the wiser for doing so. The versions of the Norse myths Snorri heard and recorded aren’t necessarily the same ones being read and circulated today. At least, not precisely. Most scholars agree that Snorri’s Prose Edda was completed around 1220, but the three primary manuscripts containing these stories that are still available to us today date between the 13th and mid-14th centuries. At some point over the course of 130 years, Snorri’s myths were collected along with various other early medieval works, and included in compilations that are sometimes abbreviated, often incomplete. It’s possible that one of these manuscripts is a direct copy of Snorri’s text—but even so, it is still a copy. Overall, there are gaps in these narratives. Missing pages. Lines of text packed densely onto vellum (in places, almost illegibly) to save precious space. These manuscripts capture the hands of different scribes at different moments. They are projections into the past reflecting Snorri’s own rearview projections. It’s impossible to say what shape the myths may have taken before these transcriptions, much less before Snorri himself wrote them down; we cannot make arguments out of silence.

Readers hoping for a fully novelistic reinvention of these myths in Norse Mythology will be disappointed. Gaiman has neither fabricated passages that might fill lacunae in existing manuscripts, nor concocted new adventures for old gods. Instead, Norse Mythology is a considered retelling of sixteen familiar tales, presented in virtually the same sequence as they are found in Snorri’s Prose Edda, and crafted as sympathetically as any modern author can. There are echoes of Ibn Fadlan’s account, for instance, in Gaiman’s description of the funeral of Odin’s second son, Balder, one of the most beautiful and beloved of the gods. Like the noble Rūs man whose body was brought to the riverside, laid in a ship there, and burned with a woman by his side, Gaiman’s Balder was brought down the shingle, and when his wife “saw her husband’s body carried past … her heart gave out in her breast, and she fell dead on to the shore. They carried her to the funeral pyre, and they placed her body beside Balder’s.”

Gaiman does, however, take some creative license—largely for the better. This is perhaps most evident in his fantastic riffs off the pun-heavy Old Norse sense of humor. Snorri’s work emphasizes Thor as a god worth admiring for brawn rather than brains; Gaiman develops this characterization, fittingly, for comedic effect. When Loki explains that the lord of ogres wants Freya’s hand in marriage, for instance, Thor thinks it’s not such a bad deal: “She had two hands, after all, and might be persuaded to give one of them without too much of an argument.”

The gods in Gaiman’s stories are also far more talkative than those in medieval versions, and readers are granted further insight into their thoughts: “The people worshipped Frey and they loved him, but this did not fill the empty place inside him.” Here, Gaiman reads Snorri’s more superficial depiction of Frey’s desires as a sign of the god’s loneliness, which offers a poignant and logical explanation for his pursuit of the giantess Gerd later in this tale.

Still, Gaiman stops short of elucidating what rituals were performed when the people worshipped the Aesir and Vanir, the two groups of gods in the Norse pantheon. By and large, his lively expansions to existing passages succeed in being true to the spirit of earlier tellings. His alterations neither fundamentally change the source material nor our understanding of it, but they may very well enhance our experience of reading it.

That these stories continue to exist at all tells readers that Snorri—like Gaiman—wanted them to endure. As Snorri explains in Skáldskaparmál, the extended lesson in poetic diction that makes up the second half of his Prose Edda, it would be a loss to forget these “ancient metaphors,” because without them the poet’s vocabulary would be diminished. Given that the trademarks of skaldic poetry are its ornate meter, difficult syntax, and often obscure kennings, it’s understandable that a writer like Snorri would want to have a wide variety of words and phrases available for reference. But Snorri also seems to relish these myths for their vivid subject matter, even as he insists that in no way are “Christian men to believe in heathen gods, nor in the truth of these tales.” His Prose Edda, one of our most full and elaborate records of pagan Norse mythology, is, in other words, far from an objective manual for religious belief (much less a guide to early pagan rites). Rather, he suggests, it is a handbook of stock images and characters for budding poets to draw upon when creating their own narratives. Though there’s little doubt that at least some of the stories he’s inherited came from older, pagan sources, readers today can never know how much Snorri changed for didactic purposes, nor how much he invented.

In Norse Mythology, Gaiman mimics the elision also found in Snorri’s own accounts, the avoidance of specifics where certainty about the old ways can’t reasonably be found or expected. Gaiman’s Odin “stood by the grave at the end of the world, and in that place he invoked the darkest runes and called on old powers, long forgotten. He burned things, and he said things, and he charmed, and he demanded.” What these things entail, not even Gaiman can say.

These tales, much like Snorri’s Edda, evoke more than they explain. They entice and enthrall. And in these (as in all) new versions of old myths, what readers encounter are the authors’ impressions of pagans and gods, tolerant and flawed and wise, shaped by good intentions. But the seeds of these stories were planted so long ago now, there’s no way of telling what branches shot up from which roots—only that the passing centuries haven’t stopped them from growing. Gaiman’s retelling adds another leaf to this ancient tree: It’s not a new species in its own right, but rather a fresh sign that the old one is still thriving.