The conceptual framework for causes presented here is intended neither as a review nor an expansion of knowledge, but rather as a viewpoint which bridges the gap between metaphysical notions of cause and basic epidemiologic parameters. The focus, then, is neither metaphysics nor epidemiology, but the gulf between them. In the same spirit as recent discussion on these pages about definitions of basic epidemiologic terms such as rate (1), common agreement on the conceptual interrelationship of causes may facilitate communication about causes of illness. A strong motivation for presenting this scheme is the often-heard confusion of two important but distinct epidemiologic issues: confounding and effect modification. These two properties of variables have different areas of relevance (2). The confounding property is not an intrinsic characteristic of any variable. Confounding, defined as distortion in an effect measure introduced by an extraneous variate, occurs only in the context of a particular study, and the same variable which confounds in one study may not confound the same association in another study setting.