The paper examines the nature of causality and how the concept can be interpreted and used in epidemiological psychiatry. The relationship of causality to logical relations is reiterated, and it is emphasized that these relationships in their pure form cannot apply to any one cause of an effect which is the outcome of several causes. In such a case the relation of the cause to the effect is one of weak implication. The fact of weak implication opens the door to the construction of certain causal models, and the possibilities afforded by this situation are illustrated by reference to the work of Brown et al (1975). Weak implication may be an intransitive relationship, and this is shown to invalidate certain ways of testing such models and the conclusions which arise from them.