Hume???s answer: (a) since past events are going to be like future events, we can be sure they will all have causes. (b) we can???t be sure: all we do is ???imagine??? events will have causes, we develop that habit. (c) there is no such thing as a future event or (for that matter) a past event, only present events. (d) it is impossible even to imagine an event without imagining it as having had a