Janus: A Summing Up is a 1978 book by Arthur Koestler, in which the author develops his philosophical idea of the holarchy. First introduced in Koestler's 1967 book, The Ghost in the Machine, the holarchy provides a coherent way of organizing knowledge and nature all together.
The idea of the holarchy is that everything we can think of is composed of holons (simultaneously both part and whole), so that each holon is always a constituent of a larger one and yet also contains other holons that are constituents of a lower level system within. Every holon is like a two-faced Janus, the Roman god: one side (the whole) looks down (or inward); the other side (the part) looks up (or outward). Each whole is a part of something greater, and each part is in turn an organizing whole to the elements that constitute it. Koestler believed that everything in a healthy system is organized this way, from the human body, to chemistry to the history of philosophy.
The concept of holon, however, is closely integrated in Janus with the theory of complex systems as was developed by Ludwig von Bertalanffy and Herbert Simon, both well known investigators and friends of Koestler. Janus put together one of the first broad based arguments for incorporating the theory of complex systems into the philosophy of science and epistemology. wikipedia 
Arthur Koestler's book "Janus: A Summing Up" was published in 1978 and presents his ideas about the nature of creativity and the role it plays in human culture and society. The main argument of the book is that creativity arises from the interaction of two different modes of thought: the "Janusian" mode and the "hierarchic" mode.
According to Koestler, the Janusian mode of thought is characterized by the ability to see things from multiple perspectives, to make unexpected connections between seemingly unrelated concepts, and to tolerate ambiguity and paradox. This mode of thought is named after the Roman god Janus, who is depicted with two faces, one looking forward and one looking back, symbolizing the ability to see both past and future, and to simultaneously hold opposing viewpoints. The hierarchic mode of thought, on the other hand, is characterized by a more linear, logical, and analytical approach. This mode of thought is focused on categorization, analysis, and synthesis, and is essential for problem-solving and decision-making. Koestler argues that creativity arises when these two modes of thought interact and complement each other. He suggests that many creative breakthroughs occur when individuals are able to shift between these modes of thought, and that the most successful creative individuals are those who are able to master both modes and use them in a complementary way. Overall, Koestler's main argument in "Janus" is that creativity is a complex and multifaceted process that involves the interaction of different modes of thought, and that understanding the nature of this process is essential for understanding human culture and society.