In fields ranging from immunology and ecology to economics and thermodynamics, multi-scale complex systems are ubiquitous. They are also notoriously difficult to model. Conventional approaches take either a bottom-up or top-down approach. But in disturbed systems, such as a post-fire forest ecosystem or a society in a pandemic, these unidirectional models can’t capture the interactions between the small-scale behaviors and the system-level properties. SFI External Professor John Harte (UC Berkeley) and his collaborators have worked to resolve this challenge by building a hybrid method that links bottom-up behaviors and top-down causation in a single theory.