In a striking departure from the Christian eschatalogy that Joyce was fed by the Catholic church, he introduces the notion of reincarnation—"a past life, "metempsychosis," "the transmigration of souls"—in both Proteus and Calypso. This conception of human immortality, evidently derived from various sources, maintains a jocoserious presence throughout Ulysses as one possible ultimate reality, and as a kind of metaphor or analogue for the mysterious interpenetration of individual identities.
As he walks on the beach Stephen imagines having been alive in the 1330s as a different "I, a changeling." Changelings, commonly encountered in Irish folklore (as in W. B. Yeats' poem The Stolen Child), are mysterious beings swapped for human children by the fairies—or, sometimes, the stolen children themselves. The belief probably arose as an explanation for birth defects, diseases, and other afflictions. It is not clear why Stephen thinks of past lives in these terms, or even that he believes he had one. He may only be imagining himself as a fairy child raised in a medieval bed as a fanciful way of indulging a historical revery. What is certain is that his ability to travel through time proceeds from some deep racial identity with earlier Irish people: "Their blood is in me, their lusts my waves."
Less than a page later, however, Stephen does explicitly think of reincarnation, whimsically imagining that the dog he sees nosing about the beach is "Looking for something lost in a past life." Still later in Proteus, he broadens the concept of transmigration from temporal to spatial extension. Somewhere out there in the innumerable "worlds" that Giordano Bruno supposed to populate the cosmos there is a parallel universe, some unknown planet in which Stephen plays a part: "Me sits there with his augur's rod of ash, in borrowed sandals, by day beside a livid sea, unbeheld, in violet night walking beneath a reign of uncouth stars."
For Bloom, the concept comes up more abstractly, when his wife asks him to explain an unfamiliar word she has encountered in a novel: "— Metempsychosis, he said, frowning. It's Greek: from the Greek. That means the transmigration of souls." The word is indeed Greek, and the belief was one feature of Orphic religion in ancient Thrace: Orpheus was said to have taught that a life of ascetic purity could liberate the soul from an otherwise endless cycle of reincarnation.
The early philosopher Pythagoras apparently brought the doctrine into greater Greek circulation, practicing vegetarianism in the belief that abstaining from harming other sentient beings could affect the soul's reincarnation in one animal form or another. Many of Plato's works suggest that he was strongly influenced by the Pythagorean doctrine (for example, the theory of recollection advanced in the Meno), and in the myth of Er at the end of the Republic he gave reincarnation a culturally influential expression. Er comes back from the dead to relate how departed human souls are judged and then choose new forms of existence, many of them animal.
When Molly asks for a definition of metempsychosis, Bloom silently racks his brain for what he knows: "That we live after death. Our souls. That a man's soul after he dies. Dignam's soul..." Impressively, he rouses his inner lecturer to give Molly a brief but coherent account: "— Some people believe, he said, that we go on living in another body after death, that we lived before. They call it reincarnation. That we all lived before on the earth thousands of years ago or some other planet. They say we have forgotten it. Some say they remember their past lives." Interestingly, Bloom too makes the connection to living on "some other planet."
Realizing that all of this will not mean much to Molly, he tries again, with unfortunate results: "— Metempsychosis, he said, is what the ancient Greeks called it. They used to believe you could be changed into an animal or a tree, for instance. What they called nymphs, for example." Metempsychosis should not be conflated with metamorphosis as Bloom does, but there is certainly overlap, as reincarnation erases the boundary between animal and human. Ulysses plays both with metamorphosis in this life (as when human beings are changed into countless animal forms in Circe) and with metempsychosis in the afterlife (as when a bat flits about Paddy Dignam's old house in Nausicaa, suggesting that Paddy may have adopted a new shape). The two conceptions share the notion that, in Bloom's words, "we go on living in another body."
It seems likely that the ancient Greeks and Romans derived their ideas of reincarnation ultimately from Indian doctrines that reach back to the Upanishads. No doubt Joyce encountered some of these more directly in theosophical writings of the late 19th and early 20th centuries. One section of Cyclops hilariously employs words covered with Sanskrit-like diacritical marks to narrate the afterlife experiences of Paddy Dignam. Such passages may easily be dismissed as comic hyperbole, but they plant the seeds of speculation.
Whatever Ulysses may be judged to say or not say about the afterlife, these quasi-eastern thoughts about reincarnation clearly do invite the reader to think about the mutability of human personality and the permeability of its boundaries. In Hades, as Dignam's coffin is diving down into the beyond and the men handling the ropes are struggling "up and out" of that dark place, Bloom thinks, "If we were all suddenly somebody else." To pursue that thought, speculations about the destination of Dignam's soul are unnecessary. Much of Circe can be read as people suddenly becoming somebody else in the here and now, and Scylla and Charybdis explores the mechanisms by which Shakespeare did so again and again in his plays.
Viewed in this way, metempsychosis is another name for self-knowledge: the ability to imagine the different forms that one's life can take. It is also a name for empathy or compassion: the ability to feel the sufferings of others as if one lived within their bodies. The word is used in this second way in Oxen of the Sun, when Bloom contemplates "the wonderfully unequal faculty of metempsychosis possessed by" the medical students, who cannot imagine the effect that their raucous laughter may be having on the unfortunate woman trying to give birth. Remembering a past life "in another body" probably has less ultimate value than being able to loosen the bonds of self imposed by inhabiting a body in this life.