A Complexity Ladder for Big History J. N. Nielsen Correspondence | J. Nick Nielsen john.n.nielsen@gmail.com Citation | Nielsen, J. N. (2024). A Complexity Ladder for Big History. Journal of Big History, VII(2); 1-8. DOI | https://doi.org/10.22339/jbh.v7i2.7202 Abstract: Complexity is a central problem for big history because big history has made complexity a central theme, constructing a cosmological periodization based on the sequential emergence of qualitatively distinct forms of complexity. How can the big historian differentiate distinct thresholds of emergent complexity while subordinating the entire sequence of thresholds to a single metric of complexity that demonstrates the increase of complexity over multiple scales of magnitude and across qualitatively distinct forms of complexity? The cosmologists’ use of a cosmic distance ladder suggests an analogous construction for complexity: a complexity ladder for big history. While no complexity ladder is formulated in this paper, the program required for a complexity ladder is sketched. Keywords: complexity, emergence, thresholds, big history, cosmological distance ladder, scientific measurement, The Problem of Complexity: Definition and Distinction Complexity is a central concept of big history, which makes use of thresholds of complexity to produce a periodization that holds from the most humble detail to the largest cosmological context. Unfortunately, there is no consensus in big history on a definition of complexity, nor on a metric for the measurement of complexity. This should not surprise us. The relative recentness of big history means that we cannot expect its fundamental concepts to be adequately defined as yet. Differences among big historians keep the nascent discipline percolating with ideas; big history is nowhere near a mature formulation such as we would expect from a well-established discipline. The absence of a clear definition and metric for complexity is a deficiency, but one that need not prevent field building in big history, but it is a deficiency of which we must be mindful, and which we should want to rectify at the earliest opportunity. Furthermore, there is an implicit tension in big history between recognizing thresholds of emergent complexity, which implies distinct kinds of emergent complexity, and the attempt (or, if the attempt hasn’t been made in any serious way, then the desire) to find a common measure for emergent complexity. If distinct emergent complexity regimes represent qualitatively different kinds of being (an ontological formulation of the problem), then these qualitatively different kinds of being ought to be measured by qualitatively distinct metrics. However, were we to measure distinct forms of complexity by distinct measures of complexity, then the pretence of a periodization constructed on the basis of the increasing complexity of emergents falls apart. This, too, like the absence of a consensus definition of complexity, need not be a disaster: the claim that the universe manifests increasing complexity can be isolated from and developed independently of the claim that the history of the universe exhibits qualitatively distinct forms of complexity; both may be true, both may be false, or either may be true independently of the other. Taking the Measure of Complexity If the differentia of big history within the genus of history is periodization through thresholds of emergent complexity, then big history sets itself at odds with the entire tradition of reductivist scientific thought by seeking formulations in terms of greater comprehensivity, and placing as much weight upon the appearance of novelty as upon the persistence of consistent foundations. Ironically, however, there remains a reductivist imperative at the heart of emergentist thought by way of the very mechanism of periodization through emergent complexity: that we must unify emergent thresholds through a shared definition of complexity—whether by thermodynamic depth (Lloyd & Pagels 1988), energy flows