 Counterfactuals play a central role in the new theory, as in the old. But the counterfactuals it employs do not simply state dependences of whether one event occurs on whether another event occurs. The counterfactuals state dependences of whether, when, and how one event occurs on whether, when, and how another event occurs. A key idea in the formulation of these counterfactuals is that of an alteration of an event. This is an actualised or unactualised event that occurs at a slightly different time or in a slightly different manner from the given event. An alteration is, by definition, a very fragile event that could not occur at a different time, or in a different manner without being a different event. Lewis intends the terminology to be neutral on the issue of whether an alteration of an event is a version of the same event or a numerically different event.