Let X0 be the price of a share of CSL Computer stock at the beginning of the current trading day. Also, let Xt be the price of a share of CSL stock at the beginning of the tth trading day in the future. Clearly, knowing the values of X0, X1, . . . , Xt tells us something about the probability distribution of Xt1; the question is, what does the past (stock prices up to time t) tell us about Xt1? The answer to this question is of critical importance in finance. (See Section 17.2 for more details.)