The problem with Scotus’s argument, then, is this. The inferences from 4 to 5, and from 6 to 7, are valid only upon the assumption that if a cause is sufficient to produce an effect no other cause is a necessary condition of that effect. But this is true only if a causally sufficient condition is such that it alone suffices to produce its effect, that is, if it is causally sufficient in the strong sense. If “causally sufficient condition” is taken in the strong sense, however, there are reasons