The chances of effects
The kind of possibility that chance measures yields a corresponding sense of not-possibly-not necessity: the sense in which it is not necessary for a radioactive atom to decay, or not to decay, in any given time. This I maintain is the sense in which a sufficient cause C necessitates the existence of an effect E: C gives E no - i.e. zero - chance of not existing. It is also the sense in which a necessary cause's non-existence necessitates the nonexistence of its effect: -C gives E a zero chance of existing.