Synchronicity designates the parallelism of time and meaning between psychic and psychophysical events, which scientific knowledge so far has been unable to reduce to a common principle. The term explains nothing, it simply formulates the occurrence of meaningful coincidences which, in themselves, are chance happenings, but are so improbable that we must assume them to be based on some kind of principle, or on some property of the empirical world. No reciprocal causal connection can be shown to obtain between parallel events, which is just what gives them their chance character. The only recognizable and demonstrable link between them is a common meaning, or equivalence. The old theory of correspondence was based on the experience of such connections-a theory that reached its culminating point and also its provisional end in Leibniz' idea of pre-established harmony, and was then replaced by causality. Synchronicity is a modern differentiation of the obsolete concept of correspondence, sympathy, and harmony. It is based not on philosophical assumptions but on empirical experience and experimentation.
— Carl Gustav Jung
Jung is doing something careful here that is easy to miss: he is not claiming synchronicity explains the world, only that causality fails to. The admission built into that distinction is the harder one — that meaning arrives in experience without the architecture we expect meaning to have. Cause-and-effect is the grammar of mastery; it tells you why something happened and, in principle, how to prevent or repeat it. Synchronicity names those moments when that grammar goes silent and coherence arrives anyway, unbidden, through what looks to every instrument like coincidence.
The lineage he draws — correspondence, sympathy, harmony, Leibniz, then the long replacement by causal science — shows how many times the human mind has tried to hold this experience and then traded it away for something more tractable. Leibniz's pre-established harmony was magnificent and utterly unworkable; causality was workable and systematically blind to half of what actually happens in a life. Jung is not suggesting we return to Leibniz. He is suggesting that experience kept producing these events whether the reigning framework had room for them or not, and that a concept honest about its own limits — "the term explains nothing" — is more faithful to the phenomena than a framework that forecloses the question by definition.
Carl Gustav Jung·The Structure and Dynamics of the Psyche·1960