The bonding or link between a cause and its effect. The ancient sceptics, subsequent occasionalists, and, most famously, Hume argue that no such link is perceptible or imaginable: we can see that events do follow one upon another, but we cannot see that they must do so, or frame any notion of the necessary connection. The idea that causation is a matter of discrete events joined by links is highly problematic: what links the event to its link? Is the link to be thought of as a third partner? What is the temporal relationship between the first event, the link, and the later event?