contemporary complexity theory, and examining various existing attempts to define ‘complexity.’ I have argued (convincingly, I hope) that none of these attempts really captures all the interesting facets of what we’re talking about when we talk about complex physical systems (like the Earth’s climate). I have not yet offered a positive view, though—I have not yet told you what I would propose to use in place of the concepts surveyed here. In Chapter Three, I shall take up that project, and present a novel account of what it means for a physical system to be complex in the relevant sense. This concept, which I will call dynamical complexity, is presented as a physical interpretation of some very recent mathematical advancements in the field of information theory. The central problem that shall occupy us in the next chapter, then, is how to transform a discussion of complexity that seems to work very well for things like messages into an account that works well for things like climate systems. My hope is that dynamical complexity offers this bridge. Once this final conceptual tool is on the table, we can start applying all of this to the problem of understanding the Earth’s climate.