‘What is a cause?’ describes why causation is vital and what causal connections are. Without causation nothing would matter to anything else. We might think of a cause as one event followed by another where if the first had not occurred, the second would not have either. But how do we know that? Test cases could be set up as used in medical randomized controlled trials. However, there are counterfactual dependence theories of causation. The key dispute seems to be between those who think the cause really does produce its effect and those who think there is nothing more than a pattern of events, with no real connection between them.