Very few, if any, causal processes are self-standing – rather, they depend on features of ‘underlying systems’: sufficiently stable physical, socio-economic, legal and cultural conditions that afford such processes (‘mechanisms’ in some New Mechanists’ terms). The underlying system is whatever it is in the world that affords the causal process – a structure exhibiting sufficient stability that affords the causal activities that occur within the process and the principles that govern them. This is illustrated by Rube Goldberg machines and examples from biochemistry, economics and deskilling in child protection. Underlying systems are helpfully understood through assessing their components and features, and the affordances associated with them, with the boundaries of the underlying system considered pragmatically.