One of David Hume's central skeptical ideas was that we could never observe causation directly. So, for example, when we see that a billiard ball hits another billiard ball causing it to move, we only see the two events occur next to each other (Hume called this "constant conjunction"), we don't actually observe any "causation". Hume was a strict empiricist, meaning that he thought all knowledge came from the senses, so this lead him to doubt that we could ever have any knowledge about causation, or know for certain if some event caused another event to occur.