ABSTRACT
Science is unable to describe the Complex Systems exhaustively. Examples are the climate and geology of our planet, the ecosystems, the living beings, the human brain, the immune system, the human societies, and the economy. Although we know the fundamental physical and chemical laws governing the behavior of the Complex Systems, we find difficulties in predicting their behavior due to computational reasons. The Natural Complexity is strictly related to the Computational Complexity. For the prediction of the dynamics of Complex Systems, we need to develop new models. For the formulation of new models, it is essential to determine the common features of Complex Systems. The first joint property is suggested by the etymology of the word “Complexity” that derives from the Latin and means “intertwined.” In fact, the Complex Systems can be described as networks. There are six principal network models that differ in their degree and cluster distributions. The second common feature of Complex Systems is that they are out-of-equilibrium. The primary factor that maintains the Earth out-of-equilibrium is the thermal radiation coming from the Sun, which is a source of energy and information for the living beings. Finally, Complex Systems exhibit emergent properties.
