Science fiction it certainly isn't, at least as that genre is generally understood. It could be described as fantasy, but that isn't really right either. It has elements of both of these but could also be classed as philosophical allegory (it has been compared to Pilgrim's Progress). It evidently was written out of its author's deep religious and metaphysical preoccupations, not to say obsessions. In other words, it is a very unusual book that defies conventional categorisation.
Voyage was first published in 1920. It was David Lindsay's first and most successful novel, which isn't saying a great deal because his literary career was largely a failure. It sold few copies initially but it has continued to impress readers over the intervening decades, including C.S. Lewis and J.R.R. Tolkien; Lewis's Out of the Silent Planet was inspired by it.
The first chapter is set in a seance at a house in Hampstead, which is a rather clumsy way of introducing the main characters, who have the unlikely names of Krag, Nightspore, and Maskull. Maskull and Nightspore have been invited to attend by another guest at the seance; Krag arrives late and uninvited and disrupts proceedings dramatically We meet a number of other people as well in this chapter, but they play no further part in the story and are therefore largely redundant.
Krag invites Maskull and Nightspore to travel with him to Tormance, a planet orbiting Arcturus, which turns out to be a double star system. (This may actually be the case, although it was not known in Lindsay's day and is still uncertain.) They set off in a crystal spaceship, piloted by Krag, from an abandoned observatory in Scotland. Maskull is asleep during the journey and awakens to find himself alone and naked in a desert on Tormance. He has acquired some new sense organs of unknown function—a theme that recurs at intervals throughout the story.
The main part of the book describes Maskull's journey through various lands in search of his companions—a journey which is also a quest for spiritual enlightenment. As he travels, he encounters individuals who present different metaphysical points of view. These meetings quite often end badly for the people concerned; Maskull himself, though an honourable man with a strong conscience, commits several murders and causes other deaths indirectly.
Eventually he learns of his own impending death. He again meets Krag, who kills him after telling him that he (Maskull) is also Nightspore. The story ends with a final revelation to Nightspore/Maskull of the truth about the world and about Krag's identity; his name on Earth, he says, is Pain.
The metaphysical position that the book is based on is a form of gnosticism. The phenomenal world is an illusion, created by someone called Crystalman, also known as Shaping. He is the evil but seductive gnostic demiurge, and the world he creates is evil, for all its beauty. The ultimate Reality is Muspel. "Crystalman's empire is but a shadow on the face of Muspel."
Considered as a novel rather than an allegory, the book suffers from a basic weakness. With the exception of the first and last chapters, events are seen entirely through Maskull's eyes, but he isn't a fully realised character. We know what he looks like—a bearded giant—but we have almost no other information about him. He is deeply emotional but his reactions to events and to people are simply summarised in a few words. We are told that he feels terror, anger, despair, and so forth, but this is simply information; we are not made to feel these emotions ourselves. His psychology is not Lindsay's main concern.
There are also logical difficulties with the narrative. Maskull is able to hold deep philosophical and theological debates with the inhabitants of Tormance; this, after all, is the main point of the book. The language barrier is quickly disposed of; the first time it arises we are told that communication is possible because of a mind-reading organ that he and the person he is talking to both possess, but subsequently this is replaced or lost, yet communication is never a problem. Lindsay simply ignores the question after touching on it initially.
Then there is the clothing question. The first person Maskull meets on Tormance helpfully provides him with a garment, but apparently no footwear. We are told about the dress of other people that Maskull encounters but again nothing is said about their feet, and we are left to suppose that they go barefoot, which is surprising view of the harshness of much of the terrain. Towards the end of the book we are told that Maskull's feet have become as hard as leather—after only four days!
Lindsay is evidently not concerned with practical details of this kind. This doesn't mean that he skimps on descriptions of Tormance, but the tone is dreamlike, with the vividness and illogicality of dreams. Strange animals appear from time to time, but there are also passing references to insects and birds, which are treated as if they were the same as those on Earth. Throughout the book Lindsay's focus is narrow; he attends closely to things that interest him but skims over others that are more peripheral.
This produces a curious blend of the real and the fantastic. The evocation of mountain landscapes—much of Tormance seems to be mountainous—is particularly vivid, but these are mountains as seen in dreams, surrealistically steep and lofty, and liable to collapse and rise up without warning. All this gives the book an impressively hallucinatory intensity.
Some have described this book as a masterpiece. If so, it is a flawed one. But coming back to it after many years, I find it undeniably impressive. I'm glad I gave it a second chance.
Note added 11 April 2021: It has occurred to me that the metaphysical scheme underlying the book could be given a modern twist in terms of the simulation hypothesis—the idea that the whole universe is no more than virtual reality created in a computer.