In which t denotes his taking a train to work and d denotes his driving. The elements of P can be deduced as follows. Row 1 represents the probability of taking the train and driving the day after (next time interval) he caught the train. Since he never takes the train on two consecutive days, the elements of this row must be zero and unity respectively. Row 2 represents the probability of taking the train and driving the day after he drove. These have equal probabilities and are therefore ½ each.