Synchronicity is the occurrence of two or more events that appear to be meaningfully related but not causally related. Synchronicity holds that such events are "meaningful coincidences". The concept of synchronicity was first defined by Carl Jung, a Swiss psychiatrist, in the 1920s. During his career, Jung gave several slightly different definitions of it.Jung variously defined synchronicity as an "acausal connecting (togetherness) principle," "meaningful coincidence," and "acausal parallelism." He introduced the concept as early as the 1920s but gave a full statement of it only in 1951 in an Eranos lecture.In 1952, he published a paper "Synchronizität als ein Prinzip akausaler Zusammenhänge" (Synchronicity – An Acausal Connecting Principle) in a volume which also contained a related study by the Nobel laureate physicist Wolfgang Pauli.Synchronicity implies 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; this contradicts the Axiom of Causality in specific cases but does not contradict causality generally.In his book Synchronicity: An Acausal Connecting Principle, Jung wrote: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.In the introduction to his book, Jung on Synchronicity and the Paranormal, Roderick Main 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.Arthur Koestler wrote extensively on synchronicity in his 1972 book The Roots of Coincidence.