Is there a difference between a complicated system and a complex system? Chapman explains the difference between complication and complexity, where he states that complication is a quantitative escalation of what is theoretically reducible to parts that can be understood; whereas, a system is said to be complex when the whole can not be fully understood by analyzing the components .
A definition of complex systems by CALResCo, a cosmopolitan non-profit organisation dedicated to promoting the wider aspects of the Complex System sciences (http://www.calresco.org/) is a system that can not be described by a single rule and whose structure exists on many scales whose characterisitcs are not reducible to a single level of description. Their defintion also includes aspects such as a system with multiple objectives, and a system which exhibits unexpected features not contained in its specfications .
I agree on the first defintion related to reducibility, but not too sure I fully agree with the latter two definitions related to multiple objectives and unexpected features. Let's take for example a camping RV, it provides multiple objectives: (1) transportation, (2) shelter, (3) dwelling, (4) recreation, can we call it a complex system? I dont think so, it can be reduced to its components and they are all well understood from an engineering stand-point. We can call it a complicated system, with various subsystems (mechanical, plumbing, electrical, structural, etc..).
So according to Chapman's definition we can say that complex systems are also complicated, but complicated system do not neccessarily have to be complex.
 G. Chapman, “The Epistemology of Complexity and Some Reflections on Symposium,” in the Science and Praxis of Complexity, S. Aida et al., Editors, United Nations University, Tokyo, 1984.
 Complexity, Artificial Life and Self-Organising Systems Glossary, located at http://www.calresco.org/glossary.htm
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