To illustrate the memoryless property, we shall suppose that X represents the number of minutes that elapse before some event occurs. According to Eq. (5.9.12), if the event has not occurred in t minutes, then the probability that the event will not occur during the next h minutes is simply _. This is the same as the probability that the event would not occur during an interval of h minutes starting from time 0. In other words, regardless of the length of time that has elapsed without the occurrence of the event, the probability that the event will occur during the next h minutes always has the same value. Theoretically, therefore, it is not necessary to consider past occurrences of an event in order to calculate probabilities for future occurrences of the event.