 although I do not intend to make any general claim about the nature of immediate [secondary] causes, I shall say what suffices for something to be an immediate cause, namely that when it is present there occurs the effect, and when it is not present … the effect does not occur. And everything that has this character with respect to another is the cause of that other … That this is sufficient for one thing to be the cause of another, seems manifest. Because if not, there remains no way of knowing something to be the cause of something else. (Ockham Ord. I dist. 45, qu. 1)