 As we saw in  §1.4  above, Lewis revised his 1973 account of causation to take account of chancy causation. Any account of causation that is based on the idea that causes increase the chances of their effects encounters two main problems: chance-increase is neither necessary nor sufficient for causation. (Case 1: The doctor reduces the patient’s very high chance of a heart attack by drastic surgery. Unfortunately, the surgery itself causes the patient to have a heart attack. The surgery lowers the chance of, but causes, the heart attack. Case 2: Billy and Suzy are throwing rocks at the bottle again. Each of their throws increases the chance of the bottle shattering, but Suzy’s throw pre-empts Billy’s. Billy’s throw raises the chance of, but does not cause, the bottle’s shattering.)