 This defense of the Common Cause Principle raises a worry, however. Many cases where we would like to apply the Common Cause Principle may also turn out to behave like time series. For example, consider Reichenbach’s example of the traveling theatre troupe  (Example 2  above). It is reasonable to expect that if the leading man is sick on a particular day, then he will be more likely to be sick on the following day—some illnesses last more than a day. And if he is exposed to a particular pathogen at one time, he may gain immunity against that pathogen in the future. If we exclude all such examples, then we run the risk of excluding many of the standard examples that are supposed to lend support to the principle.