The term « synchronicity » first appeared in 1928 and was coined by the Swiss psychoanalyst Carl Gustav Jung. After many revisions of the concept, he defined synchronicity as « a temporal coincidence of two or more events without a causal link between them and having the same or similar sense. The term is opposed to « synchronism », which refers to the simple simultaneity of two events. » 
More precisely, synchronicity is a correspondence in time between a psychic event – that is, a state of consciousness – and a physical event. The most common example is the person we haven’t heard from for a long time, who calls us exactly when we think of her. However, we will only talk about synchronicity if this event takes on a particular meaning for us, we will come back to it in the next article (online soon).
For Jung, these two events – psychic and physical – would in the end be only two aspects of the same reality. He called » Unus Mundus » this concept of unity of reality, in which :
« The strange principle of synchronicity acts in the world when certain things (…) behave as if they were the same thing, but they are not from our point of view. » 
According to Jung, synchronicities revolve around two fundamental notions : collective unconscious and archetypes. Collective unconscious represents the unconscious psyche common to humanity, i.e. the experience of all human history that we have inherited. It is a field through which information is transmitted. It would explain, for example, the phenomena of telepathy.
Archetypes are part of this field of information, like so many primordial images found in tales, legends, dreams or myths of universal literature. They are symbolic and serve as a model for us. When they are active in our psyche, archetypes unconsciously produce certain types of behaviour. The latter can be emotional or intellectual and are common to all humanity… since the beginning of time ! As a result, the archetypes are both intimate and impersonal in nature. Only if the activated archetypes in the individual psyche are related to danger, risk or fatal circumstances would synchronicities manifest themselves, according to Jung.
Before developing the notion of synchronicity, Jung became interested in the I Ching, the Book of Transformations  in 1924. This book was for him a method of exploring the unconscious. It is considered to be the oldest Chinese text, however its date of elaboration is not known with certainty : according to hypotheses it varies from the beginning of the 8th century BC to the beginning of the Christian era.
I Ching is a book of philosophy and cosmology that can also be used for divination. However bearing in mind the notion of evolution inherent in the manifestation of events. That is to say : no situation is ever fixed since it is part of the permanent movement of the universe. The Yin and Yang principles represent the incessant transformations of all aspects of life and universe.
The manifest, the real, is enshrined in these principles. The static side of the symbol that represents them, called Taijitu, is equalled only by the universal dynamics that they carry. Taijitu, motionless, represents the unity beyond the dualistic movement apparently generated by Yin and Yang.
Yin symbolizes darkness, cold, interiority, suppleness, femininity, while Yang is associated with light, heat, exteriority, rigidity, masculinity. Yin and Yang can only be defined in relation to each other : they interact in interdependence.
Our western vision presents them as opposites, whereas they are complementary. They evolve in a movement that invariably produces a passage from one to the other, offering a continuous experience of each principle. This passage is only the fruit of a mutual limitation since the Yin manifests itself as soon as the Yang reaches its maximum, and vice versa.
Thus the phases of growth and decrease of each principle alternate continuously, the growth of Yin being simultaneous and proportional to the decrease of Yang, and vice versa. We are talking about the alternative circulation of the opposites.
This shift from one principle to the other will take place gradually, leading to an inevitable changeover. Kind of like the last straw. If one observes the drip without having access to the water level in the silt, one can never know whether one is in the slow mutation phase – which causes the water level to rise gradually – or very close to overflow.
Furthermore, we will see that there are successive subdivisions of Yin and Yang. They result in a series of 64 six-pointed figures. Called hexagrams, these represent the 64 possible combinations of Yin and Yang. Thanks to them, all possible transformations can be interpreted.
Within the framework of a divination, it is then sufficient to let the unconscious knowledge emerge, which will resonate with the state of consciousness of the consultant. In Chinese thought, it is Tao that underlies the physical and psychic events involved in this process. And that’s what Jung was trying to translate when he coined the term « synchronicity » :
« Orient bases its thinking and its evaluation of facts on another principle. We don’t even have a word for it. Of course Orient has a word for it, but we do not understand it. The oriental word is Tao… I use another word for it, but it’s pretty poor. I call it synchronicity. » 
If the I Ching shows the complementarity of the Yin and Yang principles, it also reveals their successive subdivisions.
Yin and Yang can each be divided infinitely into Yin and Yang sub-elements. For example, in winter, it is the Yin principle that manifests itself more than the Yang. A winter day will be manifested by Yang in Yin. A winter day twilight will be manifested by Yin in the Yang of Yin. And we can go on and on and on forever.
This infinite division of Yin and Yang is not remembered by the Fourier transforms, which deploy a fundamental harmonic into an infinite sum of harmonic frequencies. In other words, it is reminiscent of the fractal principle.
The physicist Nassim Haramein argues that « I Ching encodes the geometry of space-time, and Yin Yang encodes the dynamics of space-time » . In his theory, there is indeed a direct analogy between the symbol of Yin and Yang and the dynamics of the universe. The Taijitu symbol represents a double torus seen from above, that is to say a black hole in which information circulates. This happens thanks to a feedback loop that acts continuously from vacuum to matter and from matter to vacuum (see the article The fractal and holographic universe). In this sense, vacuum is very close to the notion of the collective unconscious described by Jung.
For the physicist, there is also an analogy between the 64 possible combinations of Yin and Yang and the 64 tetrahedrons that describe the vacuum structure, from which everything emerges and everything returns.
The key is to interpret the I Ching literally, i.e. to observe it from the angle of geometry. Nassim Haramein starts from the following constant : the only 3D geometry that can be generated with six lines (the six lines of the hexagrams) is the tetrahedron . In order to respect the polarity of the universe, it is necessary to generate an inverse tetrahedron. Unlike the features of the first tetrahedron, the features of the second tetrahedron will have to be segmented, cut in two. Thus the features of the two tetrahedrons will be able to cross each other. This is why, according to him, the I Ching is made up of solid lines and segmented lines, and this is why each symbol is in symmetrical opposition : 1 and 64, 2 and 63, etc…
« If you take the code, if you continue this assembly, you will be able to reconstitute the whole matrix of 64 tetrahedrons. 64 codes each forming a tetrahedron, that makes a grid of 512, which represents the next fractal level after the 64 (64x8=512). So if you follow the code « I Ching » (which is the trigram of 8) we have the 64 hexagrams, which generate the 512 with all the progression. »
In the next article in this series, I propose to explore the links between synchronicities, acausality and unconscious.
 JUNG Carl Gustav, Les Racines de la conscience, Paris : Le livre de poche, 1995, p. 528, free translation
 JUNG Carl Gustav, quoted by metapsychique.org, free translation
 According to WIKIPEDIA
 JUNG Carl Gustav, quoted by metapsychique.org, op.cit., free translation
 HARAMEIN Nassim, quoted by Resonance Science Foundation
 A tetrahedron is a pyramid with a square base, considered as the fundamental building block of the universe in Nassim Haramein’s theory.