WHAT IS COMPLEXITY?
Complexity, just like for example energy, is a natural property of every system. It is defined as a function of structureand uncertainty. Humans instinctively try to stay away from highly complex scenarios because of one fundamental reason – high complexity implies a capacity to deliver surprising behavior. Since complexity is a function of entropy, it is measured in bits. Complexity is an unusual function. It combines two "antagonistic" components which in Nature tend to oppose each other. Structure attempts to persist in the face of the erosive action of entropy. Our complexity metric blends them together
Via David G Wilson