Share your insights or perspective on the financial matter presented in the input.

What does it mean that stocks are “memoryless”?
@jidugger mostly got it right. It basically mean that past performance of a stock, or a basket of stocks, are not at all useful when trying to predict its future. There is no proven correlation between past and future performance. If there was such a correlation, that was "proven" or known, then investors would quickly exploit this correlation by buying or selling this stock, thus nullifying the prediction. It doesn't mean the specific individuals cannot predict the future stock market - hell, if I set up 2^100 different robots, where every robots gives a different series of answers to the 100 questions "how will stock X do Y days from now" (for 1<=Y<=100), then one of those robots would be perfectly correct. The problem is that an outside observer has no way of knowing which of the predictor robots is right. To say that stock is memoryless strikes me as not quite right -- to   the extent that stocks are valued based on earnings, much of what we   infer about future earnings relies on past and present earnings. To put it another way - you have $1000 now, and need to decide whether to invest in a particular stock, or a stock index. The "memoryless" property means that no matter how many earning reports you view ... by the time you see them, the stock price already accounts for them, so they're not useful to you. If the earning reports are positive, the stock is already "too high" because people bought it before you did. So on average, you can't use this information to predict the stock's future performance, and are better off investing in an index fund (unless you desire extra risk that doesn't come with more profitability).