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the behavior of a system doesn’t imply that the compression scheme of biology can’t be applied to that system at all. However, removing the person from the system does render a large number of compression schemes predictively useless, whether or not they still could be applied: removing the person pushes the system into a state for which the patterns identified by (e.g.) biology and psychology don’t apply, whether or not the static carvings of those disciplines can still be made.

This fact can be generalized. The sense in which a system containing me is more complex (all other things being equal) than is a system containing my cat instead of me is just that the system containing me can be usefully carved up in more ways than the system containing my cat. My brain is more complex than my cat’s brain in virtue of there being more ways to compress systems containing my brain such that the time-evolution of those states can be reliably predicted than there are ways to compress systems containing my cat’s brain such that the same is true. The global climate today is more complex than was the global climate 1 billion years ago in virtue of there being more ways to usefully carve up the climate system today than there were 1 billion years ago[1]. Complexity in this sense, then, is a fact not about what a system is made out of, or how many parts it has, or what its shape is: it is a fact about how it behaves. It is a dynamical fact—a fact about how many different perspectives we can usefully adopt in our quest to predict how the system will change over time. One system is more dynamically complex than another if (and only if) it occupies a point in configuration space that is at the intersection of


  1. If this assertion seems suspect, consider the fact that patterns identified by economists (e.g. the projected price of fossil fuels vs. the projected price of cleaner alternative energies) are now helpful in predicting the evolution of the global climate. This was clearly not the case one billion years ago, and (partially) captures the sense in which humanity’s emergence as a potentially climate-altering force has increased the complexity of the global climate system. This issue will be taken up in great detail in Chapter Three.

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