How to define complexity?
What a newborn baby perceives is a change amidst a huge mess. Fortunately it is equipped with instincts and prior (or a priori) knowledge called here wisdom of the body (WOB) with which it simplifies this mess to meet its needs. For the baby this mess is extremely complex, it is the hallmark of complexity. Yet since the baby lacks a mind which it will develop later on, “complexity” (the concept) is meaningless to it or better not relevant. After all it has the necessary tools to make this mess meaningful.
Some babies grow to become physicists whose main concern is how to harness this change in order to build machines. In order to proceed they adopt the following concepts: The whole is the sum of its parts, every change has a unique cause. The context in which change occurs, is random and negligible. Or better one may always define a context in which the cause and effect relationship will hold. They are assisted by a special language, mathematics, with which these concepts may be studied rationally.
When physics confronted life (Descartes) and regarded it as a special kind of machine, which has a soul. The latter you need for religious purposes and since soul is not a scientific entity (not even a context) it can be ignored. Yet already Aristotle claimed that this soul controls the machine (organism) and therefore ought to be regarded as its context.
For the next centuries complexity was hardly an issue. Since the whole equals the sum of its parts, you may always reverse engineer it and discover its structure. Then came two sobering discoveries:
1. Godel’s incompleteness theorems revealed that even mathematics is incapable of encompassing any complexity. “any theory capable of expressing elementary arithmetic cannot be both consistent and complete”.
In order to make a system “complete” you need additional statements from outside the system which I regard as the context of this particular system.
2. A three body system whose parts obey Newton’s laws yet the system as a whole may become chaotic.
I regard a system as complex when the whole is more than the sum of its parts and it cannot be understood by dissecting it into its elements. The parts interact and it appears as if the whole controls them. Yet it lacks any central control.