 (i) Imperfect Regularities. The first difficulty is that most causes are not invariably followed by their effects. For example, smoking is a cause of lung cancer, even though some smokers do not develop lung cancer. Imperfect regularities may arise for two different reasons. First, they may arise because of the heterogeneity of circumstances in which the cause arises. For example, some smokers may have a genetic susceptibility to lung cancer, while others do not; some non-smokers may be exposed to other carcinogens (such as asbestos), while others are not. Second, imperfect regularities may also arise because of a failure of physical determinism. If an event is not determined to occur, then no other event can be (or be a part of) a sufficient condition for that event. The success of quantum mechanics—and to a lesser extent, other theories employing probability—has shaken our faith in determinism. Thus it has struck many philosophers as desirable to develop a theory of causation that does not presuppose determinism.