No event period has a single cause. In order to hit a pool ball with another pool ball, you need to create the universe first. It sounds pedantic, but you can't

No event period has a single cause. In order to hit a pool ball with another pool ball, you need to create the universe first. It sounds pedantic, but you can't always find more than one reasonable cause for any event. Aristotle named four, but there's always more than that.