Synchronicity (German: Synchronizität) is a concept, first introduced by analytical psychologist Carl Jung, which holds that events are "meaningful coincidences" if they occur with no causal relationship yet seem to be meaningfully related.
During his career, Jung furnished several different definitions of the term, defining synchronicity as an "acausal connecting (togetherness) principle;" "meaningful coincidence;" "acausal parallelism;" and as a "meaningful coincidence of two or more events where something other than the probability of chance is involved."
Jung's belief was that, just as events may be connected by causality, they may also be connected by meaning. Events connected by meaning need not have an explanation in terms of causality, which does not generally contradict universal causation but in specific cases can lead to prematurely giving up causal explanation.
Though introducing the concept as early as the 1920s, Jung gave a full statement of it only in 1951 in an Eranos lecture. In 1952, Jung published a paper titled "Synchronizität als ein Prinzip akausaler Zusammenhänge" ('Synchronicity - An Acausal Connecting Principle') in a volume which also contained a related study by the physicist and Nobel laureate Wolfgang Pauli, who was sometimes critical of Jung's ideas.
Jung used the concept in arguing for the existence of the paranormal. Also a believer in the paranormal, Arthur Koestler wrote extensively on synchronicity in his 1972 book The Roots of Coincidence. Moreover, it is considered that multiple meaningful coincidences contribute to the early formation of schizophrenic delusions (see also: apophenia), distinguishing which of these synchronicities can be morbid, according to Jung, is a matter of interpretation - pathology, if any, lies in the reaction rather than in occurrence of synchronistic (low probability but normal) event experience.
As it is neither testable or falsifiable (see: scientific method), synchronicity does not fall into the realm of empirical study . Main objection from scientific standpoint is that synchronistic events are experimentally indistinguishable from ordinary coincidences. Almost any coincidence can have meaning dependent on observer's various intuitive interpretation. Mainstream science explains synchronicities and even "mere coincidences" as underestimated chance events or spurious correlations which can be described by laws of statistics (e.g. by the law of truly large numbers) and confirmation biases. However, for lack of more sophisticated explanations coincidence can also be useful as kind of link to folk psychology and philosophy.
The following past ideas and sources could have contributed to the creation of the concept:
How are we to recognize acausal combinations of events, since it is obviously impossible to examine all chance happenings for their causality? The answer to this is that acausal events may be expected most readily where, on closer reflection, a causal connection appears to be inconceivable.... It is impossible, with our present resources, to explain ESP [extrasensory perception], or the fact of meaningful coincidence, as a phenomenon of energy. This makes an end of the causal explanation as well, for "effect" cannot be understood as anything except a phenomenon of energy. Therefore it cannot be a question of cause and effect, but of a falling together in time, a kind of simultaneity. Because of this quality of simultaneity, I have picked on the term "synchronicity" to designate a hypothetical factor equal in rank to causality as a principle of explanation.
Roderick Main, in the introduction to his 1997 book Jung on Synchronicity and the Paranormal, wrote:
The culmination of Jung's lifelong engagement with the paranormal is his theory of synchronicity, the view that the structure of reality includes a principle of acausal connection which manifests itself most conspicuously in the form of meaningful coincidences. Difficult, flawed, prone to misrepresentation, this theory none the less remains one of the most suggestive attempts yet made to bring the paranormal within the bounds of intelligibility. It has been found relevant by psychotherapists, parapsychologists, researchers of spiritual experience and a growing number of non-specialists. Indeed, Jung's writings in this area form an excellent general introduction to the whole field of the paranormal.
Jung felt synchronicity to be a principle that had explanatory power towards his concepts of archetypes and the collective unconscious.[i] It described a governing dynamic which underlies the whole of human experience and history--social, emotional, psychological, and spiritual. The emergence of the synchronistic paradigm was a significant move away from Cartesian dualism towards an underlying philosophy of double-aspect theory. Some argue this shift was essential in bringing theoretical coherence to Jung's earlier work.[ii]
Even at Jung's presentation of his work on synchronicity in 1951 at an Eranos lecture, his ideas on synchronicity were evolving. On Feb. 25, 1953, in a letter to Swiss author and journalist Carl Seelig, who wrote a biography of Albert Einstein, Jung wrote:
Professor Einstein was my guest on several occasions at dinner.... These were very early days when Einstein was developing his first theory of relativity [and] It was he who first started me on thinking about a possible relativity of time as well as space, and their psychic conditionality. More than 30 years later the stimulus led to my relation with the physicist professor W. Pauli and to my thesis of psychic synchronicity.
Jung believed life was not a series of random events but rather an expression of a deeper order, which he and Pauli referred to as Unus mundus. This deeper order led to the insights that a person was both embedded in a universal wholeness and that the realisation of this was more than just an intellectual exercise, but also had elements of a spiritual awakening. From the religious perspective, synchronicity shares similar characteristics of an "intervention of grace." Jung also believed that in a person's life, synchronicity served a role similar to that of dreams, with the purpose of shifting a person's egocentric conscious thinking to greater wholeness.
The occurrence of a meaningful coincidence in time can take three forms:
a) the coincidence of a certain psychic content with a corresponding objective process which is perceived to take place simultaneously.
b) the coincidence of a subjective psychic state with a phantasm (dream or vision) which later turns out to be a more or less faithful reflection of a "synchronistic," objective event that took place more or less simultaneously, but at a distance.
c) the same, except that the event perceived takes place in the future and is represented in the present only by a phantasm that corresponds to it.-- Carl Jung, "Résumé", Synchronicity: An Acausal Connecting Principle (1960)
Jung tells the following story as an example of a synchronistic event in his book Synchronicity:
By way of example, I shall mention an incident from my own observation. A young woman I was treating had, at a critical moment, a dream in which she was given a golden scarab. While she was telling me this dream I sat with my back to the closed window. Suddenly I heard a noise behind me, like a gentle tapping. I turned round and saw a flying insect knocking against the window pane from outside. I opened the window and caught the creature in the air as it flew in. It was the nearest analogy to a golden scarab that one finds in our latitudes, a scarabaeid beetle, the common rose-chafer (Cetonia aurata), which contrary to its usual habits had evidently felt an urge to get into a dark room at this particular moment.
It was an extraordinarily difficult case to treat, and up to the time of the dream little or no progress had been made. I should explain that the main reason for this was my patient's animus, which was steeped in Cartesian philosophy and clung so rigidly to its own idea of reality that the efforts of three doctors-I was the third-had not been able to weaken it. Evidently something quite irrational was needed which was beyond my powers to produce. The dream alone was enough to disturb ever so slightly the rationalistic attitude of my patient. But when the "scarab" came flying in through the window in actual fact, her natural being could burst through the armor of her animus possession and the process of transformation could at last begin to move.
French writer Émile Deschamps claims in his memoirs that, in 1805, he was treated to some plum pudding by a stranger named Monsieur de Fontgibu. Ten years later, the writer encountered plum pudding on the menu of a Paris restaurant and wanted to order some, but the waiter told him that the last dish had already been served to another customer, who turned out to be de Fontgibu. Many years later, in 1832, Deschamps was at a dinner and once again ordered plum pudding. He recalled the earlier incident and told his friends that only de Fontgibu was missing to make the setting complete--and in the same instant, the now-senile de Fontgibu entered the room, having got the wrong address.
After describing some examples, Jung wrote: "When coincidences pile up in this way, one cannot help being impressed by them - for the greater the number of terms in such a series, or the more unusual its character, the more improbable it becomes.":91
In his book Thirty Years That Shook Physics - The Story of Quantum Theory (1966), George Gamow writes about Wolfgang Pauli, who was apparently considered a person particularly associated with synchronicity events. Gamow whimsically refers to the "Pauli effect," a mysterious phenomenon which is not understood on a purely materialistic basis, and probably never will be. The following anecdote is told:
It is well known that theoretical physicists cannot handle experimental equipment; it breaks whenever they touch it. Pauli was such a good theoretical physicist that something usually broke in the lab whenever he merely stepped across the threshold. A mysterious event that did not seem at first to be connected with Pauli's presence once occurred in Professor J. Franck's laboratory in Göttingen. Early one afternoon, without apparent cause, a complicated apparatus for the study of atomic phenomena collapsed. Franck wrote humorously about this to Pauli at his Zürich address and, after some delay, received an answer in an envelope with a Danish stamp. Pauli wrote that he had gone to visit Bohr and at the time of the mishap in Franck's laboratory his train was stopped for a few minutes at the Göttingen railroad station. You may believe this anecdote or not, but there are many other observations concerning the reality of the Pauli Effect! 
Causality, when defined expansively (as, for instance, in the "mystic psychology" book The Kybalion, or in the platonic Kantian Universal causation), states that "nothing can happen without being caused." Such an understanding of causality may be incompatible with synchronicity. In contrast, other definitions of causality (e.g., the neo-Humean definition) are concerned only with the relation of cause to effect, and are thus more compatible with synchronicity. There are also opinions that hold cause to be internal when there is no external observable cause.
It is also pointed out that, since Jung took into consideration only the narrow definition of causality--only the efficient cause--his notion of acausality is also narrow and so is not applicable to final and formal causes as understood in Aristotelian or Thomist systems. Either the final causality is inherent in synchronicity, as it leads to individuation; or synchronicity can be a kind of replacement for final causality. However, such finalism or teleology is considered to be outside the domain of modern science.
Jung's theory of synchronicity is nowadays regarded as pseudoscientific, as it is not based on experimental evidence, and its explananda are easily accounted for by our current understanding of probability theory and human psychology.
Jung and his followers (e.g., Marie-Louise von Franz) share in common the belief that numbers are the archetypes of order, and the major participants in synchronicity creation. This hypothesis has implications that are relevant to some of the "chaotic" phenomena in nonlinear dynamics. Dynamical systems theory has provided a new context from which to speculate about synchronicity because it gives predictions about the transitions between emergent states of order and nonlocality. This view, however, is not part of mainstream mathematical thought.
Mainstream mathematics argues that statistics and probability theory (exemplified in, e.g., Littlewood's law or the law of truly large numbers) suffice to explain any purported synchronistic events as mere coincidences. The law of truly large numbers, for instance, states that in large enough populations, any strange event is arbitrarily likely to happen by mere chance. However, some proponents of synchronicity question whether it is even sensible in principle to try to evaluate synchronicity statistically. Jung himself and von Franz argued that statistics work precisely by ignoring what is unique about the individual case, whereas synchronicity tries to investigate that uniqueness.
In psychology and cognitive science, confirmation bias is a tendency to search for or interpret new information in a way that confirms one's preconceptions, and avoids information and interpretations that contradict prior beliefs. It is a type of cognitive bias and represents an error of inductive inference, or is a form of selection bias toward confirmation of the hypothesis under study, or disconfirmation of an alternative hypothesis. Confirmation bias is of interest in the teaching of critical thinking, as the skill is misused if rigorous critical scrutiny is applied only to evidence that challenges a preconceived idea, but not to evidence that supports it.
Charles Tart sees danger in synchronistic thinking: "This danger is the temptation to mental laziness.... [I]t would be very tempting to say, 'Well, it's synchronistic, it's forever beyond my understanding,' and so (prematurely) give up trying to find a causal explanation."
Upon initial publication, the work of Jung, such as The Interpretation of Nature and the Psyche, were received as problematic by his fellow psychologists. Fritz Levi, in his 1952 Neue Schweizer Rundschau (New Swiss Observations) review, critiqued Jung's theory of synchronicity as vague in determinability of synchronistic events, saying that Jung never specifically explained his rejection of "magic causality" to which such an acausal principle as synchronicity would be related. He also questioned the theory's usefulness.
In psychology and sociology, the term apophenia is used for the mistaken detection of a pattern or meaning in random or meaningless data. Skeptics, such as Robert Todd Carroll of the Skeptic's Dictionary, argue that the perception of synchronicity is better explained as apophenia. Primates use pattern detection in their form of intelligence, and this can lead to erroneous identification of non-existent patterns.
A famous example of this is the fact that human-face recognition is so robust, and based on such a basic archetype (i.e., two dots and a line contained in a circle), that human beings are very prone to identify faces in random data all through their environment, like the "man in the moon," or faces in wood grain, an example of the visual form of apophenia known as pareidolia.
Many people believe that the Universe, angels, other spirits, or God cause synchronicity. Among the general public, divine intervention is the most widely accepted explanation for these meaningful coincidences.
Research on the processes and effects of synchronicity is a subfield of psychological study. Modern scientific techniques, such as mathematical modeling, were used to observe chance correlations of synchronicities with Fibonacci time patterns.
As far as methodology is concerned, all empirical methods can be used to study synchronicity scientifically: quantitative, qualitative, and combination methods. Most studies of synchronicity, however, have been limited to qualitative approaches, which tend to collect data expressed in non-mathematical representations such as descriptions, placing less focus on estimating the strength and form of relationships.
On the other hand, skeptics (e.g. most psychologists) tend to dismiss the psychological experience of coincidences as just yet one more demonstration of how irrational people can be. Irrationality in this context means an association between the experience of coincidences and biased cognition in terms of poor probabilistic reasoning and a propensity for paranormal beliefs.
A survey (with 226 respondents) of the frequency of synchronicity in clinical settings found that 44% of therapists reported synchronicity experiences in the therapeutic setting; and 67% felt that synchronicity experiences could be useful for therapy. The study also points out ways of explanations of synchronicity:
For example, psychologists were significantly more likely than both counsellors and psychotherapists to agree that chance coincidence was an explanation for synchronicity, whereas, counsellors and psychotherapists were significantly more likely than psychologists to agree that a need for unconscious material to be expressed could be an explanation for synchronicity experiences in the clinical setting.
Philip K. Dick makes reference to, "Pauli's synchronicity," in his 1963 science-fiction novel, The Game-Players of Titan, in reference to pre-cognitive psionic abilities being interfered with by other psionic abilities such as psychokinesis: "an acausal connective event."