 Mackie (1974: 63) conceives of each cluster of factors as a conjunction, for example \(C \land \neg A\), where \(C\) is a type of event and \(\neg A\) is the absence of any event of type \(A\). Consider, for instance, a house which burns to the ground. There is a variety of distinct clusters which can cause the burning of the house. One cluster includes a short-circuit, the presence of oxygen, the absence of a sprinkler system, and so on. Another cluster includes an arsonist, the use of petrol, again the presence of oxygen, and so on. Yet another cluster includes a burning tree falling on the house, and so on. Each cluster is a conjunction of single factors. The disjunction of all clusters represents then the plurality of causes (see Mackie 1965: 245 and 1974: 37–38 for details).