 [What happens] has the possibility of happening and the possibility of not happening. It is not insofar as its happening is possible that it exists. And it is not only insofar as something else has the possibility of making it happen that it exists from another. Having the possibility of making a thing happen is not sufficient for making it happen. Given the mere possibility of making a thing happen, which is not sufficient, the thing would sometimes exist when it exists and sometimes not exist when it exists. […] But something that has the very same relationship to a thing’s existing from it as to its not existing from it has no greater claim to be the thing’s cause than not to be its cause. Indeed, sound reason demands that there be a state that differentiates between the thing’s existing from it and its not existing from it. (Avicenna MH: 126–7, trans. modified)