 Hence, we should understand Leibniz’s statement in a letter to Arnauld (May 1686) that “there is always something to be conceived in the subject which provides the explanation why this predicate or this event belongs to it, or why a particular event happened rather than not” (GP ii, 45/PM 60) not in terms of some ideal event contained in the subject as carrying the correlative explanatory role, but the power or principle of change of the subject.