There are many theories on causation. In 1973, David Lewis thought that causality was like a chain of things depending on one another. So, for some event C to cause some event E, that means there is a chain of other events that connect them together. Other theories use probability. For example, smoking does not always cause cancer, but it increases the chances of it happening. As a result, it is more useful to turn to a definition of causality that uses probability.