 On the Millian Regularity Theory, a cause is nomologically sufficient for its effect: there is a law of nature such that the occurrence of a cause suffices for the occurrence of its effect. A cause is taken to be a totality of positive and negative factors which is sufficient and necessary for the effect. However, one and the same effect can have many causes understood as sufficient and necessary totalities (as has already been observed by Venn 1889). Each of these distinct totalities, or clusters of factors, is sufficient to bring about the effect, but no single one is necessary if another cluster is actual.