The concept of emergent properties originates in the nineteenth century, finding the primary roots back in
Kantian philosophy. The term was coined by G. H. Lewis as far back as 1875. A common definition from that time states that ''emergence is the denomination of something new which could not be predicted from the elements constituting the preceding condition.''
Throughout the last century, several scientists have addressed the concept from a more philosophical point of view, resulting in the appearance of different descriptions and explanations. The definitions have, in general, been referring to subjective arguments, such as surprise, unex-pectancy, thus being clearly observer dependent. This has strongly influenced the present approaches, and this comprehension has often been connected with a flavor of mysticism. Thus, the seriousness of the concept has often been underestimated.
During the last decades, the use of the term emergent properties has found widespread use in biological sciences, especially, because it is clearly connected with the growing implementation of the system approach in ecology. The need for a holistic concept was due to the failure of the traditional reductionistic research strategies to explain the properties of ecosystems by the knowledge of the behavior and the properties of the ecosystem constituents alone. Ecosystems are highly complex middle-numbered systems dominated by nonlinear relationships between their constituents. In such systems, things are bound to happen that are not easy to predict from the basic knowledge of the system, no matter how extensive this knowledge is.
Highly relevant to biology and ecology is the question when an emergent property appears. This leads to the distinction of 'primary' and 'secondary' emergence, primary emergence being the first time an emergent property appears. To be conserved the property can be reproduced again and again but in this case it is nominated as a secondary emergence. Recent approaches to emergence have come up with three further notions of emergence: 'computational emergence', 'thermodynamic emergence', and 'emergence relative to a model'. The computational emergence deals with the patterns produced by different computer programs, for example, cellular automata systems developing complex distributions out of simple rules from game theory. Thermodynamic emergence covers the establishment of highly complex, self-organized structures and their relations to the nonlinear, far from equilibrium thermodynamics. Emergence relative to a model defines emergence as the deviation of the actual behavior of a physical system in comparison with an observer's model of it.
Summarizing these historical notions of emergence, the following features can be stated:
• Emergent properties are properties of a system which are not possessed by component subsystems alone.
• The properties emerge as a consequence of the interactions within the system.
• Two fundamental types of interactions are found that may be characterized as intra- and inter-connectedness, that is, connections within and between levels, including controls. This point does not consider the direction of the intra-level interactions. Emergence is based on both, upward and downward causation.
• The historically emerged properties are considered 'new' with reference to their primary appearance.
• These new properties appear at one level of a system and are not immediately deducible from observation of the levels or units of which the system consists.
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