We develop a naturalised account of planning, which identifies a class of functions (and their associated behaviours) in intelligent systems. The account identifies three principal components of a planning process: a system (defined by a set of possible system-environment decompositions); a subsystem (which presents a model, copy, or analog of some aspect of the system); and a selection mechanism (via which a subsystem is functionally related to expected future states). We give a generalised, system-independent account of planning, and then ground our analysis with a set of eight concrete reference systems, spanning biological, human, social, and artificial systems. Finally, we apply this naturalized account of planning to evaluate under what conditions planning behaviour is likely to emerge, and what failure modes arise in systems exhibiting such planning behaviour.