Editorial for the special issue on 'Emergent properties of complex systems' - Emergence and levels of abstraction

The notions of emergence and emergent properties have a long history in science, and have recently regained popularity in systems science fuelled largely by the growth of computer simulation as an exploratory and investigative tool. Unfortunately, the notions and terms are not especially well defined: ideas of evolution, self-organization, collective ('systemic') properties and cooperative behaviour are all involved to a greater or lesser extent. It is of ten claimed that emergent properties arise at a particular level of system description by virtue of the interaction of relatively simple lower-level components - between themselves and with the environment - but cannot be explained at this lower level. Yet there are obvious scientific and philosophical problems with a definition based on an inability to explain observable effects in particular terms. This editorial outlines the history of emergence as a scientific concept, and reviews attempts to rer ne and qualify the term.