 (iv) Spurious Regularities. Suppose that a cause is regularly followed by two effects. Here is an example from Jeffrey (1969): Suppose that whenever the barometric pressure in a certain region drops below a certain level, two things happen. First, the height of the column of mercury in a particular barometer drops below a certain level. Shortly afterwards, a storm occurs. This situation is shown schematically in Figure 1. Then, it may well also be the case that whenever the column of mercury drops, there will be a storm. If so, a simple regularity theory would seem to rule that the drop of the mercury column causes the storm. In fact, however, the regularity relating these two events is spurious. The ability to handle such spurious correlations is probably the greatest source of attraction for probabilistic theories of causation.