 The third reason for optimizing generality of the kernel is a bit more intricate. Suppose a brick of four kilograms is thrown against a window such that the window shatters. Then, for this effect to be brought about, it is often not necessary that the brick has a weight of exactly four kilograms. A brick that is slightly lighter and thrown with the same velocity would also do. Nor does it have to be a brick, as we know from the rock-throwing examples. So it seems more accurate to say that the shattering of the window was caused by the fact or event that a rigid, non-elastic, and sharp object was thrown against the window in such a manner that the object’s impact was within a certain interval. The latter description is obviously more general than the former in the sense that more physical systems satisfy it. In brief, the kernel for \(E\) should not contain aspects of causally relevant factors that do not make a difference to \(E\).