 Suppose \(C_1, C_2,\) and \(D_1\) occur independently of one another. There is neither a strict positive nor a strict negative correlation among these events. There are cases where \(C_1\) does not occur but \(D_1\) does, and so \(E\) will occur. There are also cases where \(C_1\) occurs but \(C_2\) and \(D_1\) do not, and so \(E\) will not occur. And there will be cases where \(C_1\) and \(C_2\) occur together so that an instance of \(C_1\) qualifies as a cause of an instance of \(E\). An event of type \(C_1\) may therefore be a cause of an event of type \(E\), without there being a strict regular succession between \(C_1\)-events and \(E\)-events. This being said, Hume (1739: book I, part III, section XV) considers causation without strict regular succession. There, he seems to propose a regularity theory that goes beyond HRT and may cover causation based on non-strict succession.