How to tell if something is a synchronicity or just coincidence?

The honest answer is that you cannot always tell — and Jung knew this. The distinction is not a clean logical test but a judgment made under conditions of irreducible ambiguity, and the soul's tendency to find meaning where it wants to find it is precisely the shadow side of the question.

Still, the tradition offers several criteria that do real diagnostic work.

Meaning, not probability. The first criterion is the one Jung kept returning to: a synchronistic event is defined not by its improbability alone but by the meaning it carries. As he formulated it in Memories, Dreams, Reflections:

Synchronicity is a term coined by Jung to designate the meaningful coincidence or equivalence of a psychic and a physical state or event which have no causal relationship to one another.

Improbability is a necessary but not sufficient condition. Jung recorded a run of six fish-related events in a single day — fish for lunch, an April fish joke, a patient's fish paintings, embroidered sea-monsters, a dream of a fish — and explicitly refused to call it synchronistic, noting that he had been working on the fish symbol for months and that fish on Friday is ordinary. The numinous impression a series makes grows with its length, but impression is not evidence. The question is whether the inner and outer events share a meaning that neither caused the other to carry.

Archetypal activation. The second criterion is more specific. Von Franz, following Jung closely, observed that synchronistic events cluster around moments when an archetype has been constellated in the unconscious — states of high emotional tension, threshold experiences, crises, deaths, major transitions. As von Franz writes in Psyche and Matter:

These processes sometimes seem already to have established an "order" or "system" before they come above the threshold of consciousness. They seem to depend on the activation of an archetypal pattern.

A coincidence that arrives during an ordinary Tuesday afternoon, when nothing particular is at stake, is more likely to be mere coincidence. A coincidence that arrives at a threshold — a death in the family, a crisis of vocation, the moment a patient's resistance finally breaks — is more likely to be synchronistic, because the archetypal field is already energized. Jung's scarab arrived precisely when his patient's rationalism had sealed her off from everything that might help her; the beetle's timing was the point.

The quality of the correspondence. Tarnas, working in the Jungian lineage, describes a useful phenomenological sequence: early synchronicities tend to be ambiguous and easily dismissed; the ones that mark genuine thresholds are "unambiguous in their coincidental force and precision of patterning." The precision matters. A vague thematic resemblance between an inner state and an outer event is not the same as the exact correspondence Jung found when the beetle matched the golden scarab of the dream in species, color, and timing. The more specific the formal parallel — the more the outer event could serve as a literal translation of the inner image — the harder it becomes to attribute the coincidence to chance alone.

The shadow of the question. Here is where the diagnostic frame becomes necessary. The soul running on the ratio of desire — if I obtain the thing I most long for, I will not suffer — is exquisitely motivated to read coincidences as signs. The soul running on the pneumatic ratio — if I am spiritual enough, I will not suffer — is motivated to find the universe speaking to it personally, confirming its specialness. Both logics produce a kind of inflation that Jung explicitly named as synchronicity's shadow: the pre-Copernican error of centering the world of meaning on the old narrow self. Tarnas puts it plainly — discriminating synchronistic events requires "a self-critical awareness of unconscious tendencies towards narcissistic distortion by which random or peripheral events are continually transformed into signs of an egocentric universe." The question to ask is not only does this feel meaningful? but what would it mean for me if it were meaningful? If the answer is that it would confirm something you already desperately want confirmed, the coincidence deserves more scrutiny, not less.

What this leaves you with. No algorithm resolves the question. What the tradition offers instead is a set of conditions that raise or lower the probability of genuine synchronicity: archetypal activation, emotional intensity at a threshold, formal precision of correspondence, and the absence of obvious motivated reasoning. When all four converge, the event earns the name. When one or more is missing — especially the last — the more honest word is coincidence, and the soul's insistence that it must be more is itself the thing worth examining.

synchronicity — the acausal connecting principle, its ontological ground in the psychoid archetype, and its relation to the unus mundus
Marie-Louise von Franz — portrait of Jung's closest collaborator and the thinker who carried the synchronicity hypothesis furthest
the objective psyche — the autonomous field from which synchronistic events erupt, and why it matters that the psyche is not identical with the ego
shadow — the self-inflating and paranoid distortions that constitute synchronicity's shadow side

Sources Cited

Jung, C.G., 1963, Memories, Dreams, Reflections
Jung, C.G., 1960, The Structure and Dynamics of the Psyche
von Franz, Marie-Louise, 2014, Psyche and Matter
Tarnas, Richard, 2006, Cosmos and Psyche