This document discusses the concept of variation in causal discovery. It argues that causal reasoning relies on the identification of differences or variations, and that establishing causes requires observing how changing factors affect outcomes. The document outlines three types of causal reasoning - ordinary, experimental, and statistical - and explains how each searches for variations. It discusses philosophers John Stuart Mill and Emile Durkheim's views on the importance of variations and differences in causal analysis. The key point is that causal discovery fundamentally involves reasoning about variations in order to identify putative causes and explain phenomena.