A patient is telling Carl Jung about her dream of a golden scarab when something starts tapping at the window. He opens it, catches a rose chafer beetle as it flies in, and presents it to her as her scarab.
That is the founding anecdote of synchronicity, and it has been retold so often that it is easy to miss how odd the claim built on it was. Jung did not say the dream caused the beetle, or the beetle the dream. He said the two were connected by meaning alone, and that this kind of connection deserved a place in the description of nature alongside cause and effect.
A word for the uncanny
Jung first used the term in a 1930 lecture, a memorial address for his friend Richard Wilhelm, the translator of Chinese classics. He then sat on the idea for two decades. The full statement came in 1952, in an essay titled "Synchronicity: An Acausal Connecting Principle."
His definition is narrow. A synchronicity is a coincidence in time of two or more events that are not causally related but share a meaning for the person experiencing them, typically an inner state (a dream, a thought, a premonition) matched by an outer event. Jung proposed it as a hypothetical explanatory factor equal in rank to causality.
The scarab case shows why a therapist cared. By Jung's account the patient was relentlessly rational, and stuck. The beetle (a rose chafer, Cetonia aurata, the nearest thing to a golden scarab that a Swiss garden can offer) broke through her defenses, and treatment finally began to move. Whatever else it was, the coincidence was clinically useful.
Jung wanted more than usefulness. The essay gathers historical material, draws on the parapsychology experiments of J. B. Rhine at Duke University, which Jung followed closely, and reports an astrological experiment of his own. He believed that meaningful coincidences occur more often than chance allows.
Enter the physicist
The most surprising thing about the 1952 essay is the company it kept. It was published in a joint volume, later translated as The Interpretation of Nature and the Psyche, alongside an essay on Johannes Kepler by Wolfgang Pauli.
Pauli was not a fringe figure. He was a theoretical physicist whose work would win the 1945 Nobel Prize. In 1932, already famous and in personal crisis after a string of painful events, he turned to Jung for help. Jung referred him to an assistant, and Pauli recorded more than 400 dreams, some of which Jung later analyzed anonymously in Psychology and Alchemy.
The therapy ended. The conversation did not. The two men corresponded from 1932 to 1958, and the surviving letters, about 80 of them, were eventually published as Atom and Archetype. They discussed dreams and their relation to reality, and whether physics and psychology might be complementary views of a single underlying order.
Pauli's side of the book argued that Kepler's science was shaped by archetypal images, and set him against the Hermetic physician Robert Fludd. It has had its own critics. The historian Robert Westman, writing in 1984, pointed out that scholars had accepted the essay largely on Pauli's authority without asking why a physicist was writing history in Jungian code, or whether the history was right.
Physicists also enjoyed a running joke called the Pauli effect, the claim that laboratory equipment broke whenever he came near. Stories of this kind, including one about a vase shattering as he entered a room at Jung's institute, belong to anecdote, not evidence.
What the statisticians say
The scientific objections to synchronicity are not subtle, and they have not changed much in seventy years.
The first is arithmetic. In a much cited 1989 paper in the Journal of the American Statistical Association, Persi Diaconis and Frederick Mosteller named what they called the law of truly large numbers: "With a large enough sample, any outrageous thing is likely to happen." In a country of 250 million people, they noted, truly rare events are bound to be plentiful. One analyst hears thousands of dreams over a career. One of them will eventually line up with a bug.
The second is psychology. Diaconis and Mosteller list several other sources of apparent coincidence: hidden causes nobody has noticed, the quirks of memory and attention, and the habit of counting near misses as hits. A rose chafer is not a scarab, strictly speaking, and nobody remembers the dreams that no beetle answered. Their conclusion is that coincidences occur in the minds of observers.
The third is the charge laid in The Skeptic's Dictionary: synchronicity is a dignified name for apophenia, the human tendency to find pattern and significance in random data. Its deeper complaint is that Jung offers no objective test for which coincidences count as meaningful. If the only criterion is that someone felt it mattered, the hypothesis cannot fail, and a hypothesis that cannot fail is not doing scientific work.
What survives
It would be tidy to stop there, but the concept has had a long afterlife in exactly the place it started. A 2016 study found that 70 percent of therapists surveyed agreed that experiences of synchronicity could be useful in therapy. That is a claim about people, not about physics. A coincidence that makes someone stop and reconsider their life has done something real, whatever its probability.
This suggests where the line falls. As a principle of nature, synchronicity has no evidence behind it that survives the statisticians. As a description of experience, it names something nearly everyone has felt and few vocabularies handle well: the moment when the world seems, briefly, to be paying attention.
The beetle at the window
Diaconis and Mosteller and Jung agree on one thing. All of them locate the meaning in the observer. They differ on whether that is the end of the matter or the beginning.
Pauli, who knew better than most how strange the physical world already is, thought the question worth a quarter century of letters. He never claimed an answer. The honest position is probably his: keep the mathematics strict, keep the window open, and do not confuse the two.