Jung's founding case: a patient, stiff and rational, recounts a dream of being given a golden scarab; at that moment a scarabeid beetle taps at the window, gold-green, and Jung hands it to her, the irrational breaking the rationalization open. The concept matured in letters with Wolfgang Pauli, 1930 to 1958, and their 1952 joint volume: Jung's Synchronicity: An Acausal Connecting Principle beside Pauli's essay on Kepler. Pauli connected it to Rhine's parapsychology experiments, and the volume quietly outed him as the source of Jung's own dream series, a fact concealed for decades.
The scientific critique is structural: synchronicity cannot be tested, since any coincidence can be declared meaningful after the fact, and the ones that fail to match are never counted. Apophenia, selection bias, and the law of large numbers cover the cases without metaphysics. Jung's defenders answer that he never claimed causation, only that meaning is a real relation, and that the dismissal confuses unfalsifiable with false. Both sides are documented; the concept sits exactly at the border it describes.
The claim: synchronicity's real proposal is that events have three coordinates, cause, time, and meaning, and science's monopoly on the first two blinded it to the third, which is why the meaningful coincidence feels like a message: it is the only channel of connection we are forbidden to measure. Verdict: poetic, unfalsifiable, and clinically real, therapists use it because it works on patients, not because it survives trials. The archive's note: the scarab was at the window. Whether the window or the meaning came first is the question Jung spent thirty years not answering.
If meaning can connect events without cause, who writes the connections?