 This suggests that the regularity for an effect is a disjunction of conjunctions which is necessary and sufficient for the effect relative to a causal field. Relative to this field, each cluster is sufficient for the burning of the house, but no single one is necessary; for another cluster can be sufficient for the burning. (\(C\) is sufficient for \(E\) means that, whenever an event of type \(C\) occurs, so does an event of type \(E\). \(C\) is necessary for \(E\) means that an event of type \(E\) only occurs if an event of type \(C\) occurs.) For simplicity, suppose the complex regularity (relative to a particular causal field) has the form: