Conceptualising the social world is a critical first step in attempting to understand the aetiological role of social factors. This chapter draws on ideas developed in a previous era to suggest a theoretically informed rubric for conceptualising components of the social world to make them amenable to investigation, the focus being specifically on aetiology. The ecological notion of aetiology held by Faris and Dunham and their contemporaries eventually gave way to a focus on what they termed constitutional factors. The chapter addresses the necessity of considering distinctions between processes and conditions, levels of organisation and place and time. Care must be taken to guard against two common inferential mistakes involving multiple levels of organisation. The ecological fallacy is familiar to most investigators. The equally important converse fallacy, sometimes referred to as the atomistic fallacy, is less familiar. It results from comparing individuals and inferring causation at the group level.