This document discusses the rationale of causality in causal modeling, specifically focusing on the notion of measuring variations. It argues that measuring variations is the principle that guides causal reasoning in causal modeling based on empirical, methodological and philosophical arguments. The rationale of causality in causal modeling centers around identifying and interpreting variations in variables, which is supported by foundational thinkers like Mill, Durkheim, and Quetelet who employed comparative and concomitant variation methods. Objections regarding regularity and invariance are addressed, and methodological consequences for different types of variations are explored.