[Shorter variant:] In the short run, the market is a voting machine but in the long run, it is a weighing machine. - Benjamin Graham

The investor would not be far wrong if this motto read more simply: "Never buy a stock immediately after a substantial rise or sell one immediately after a substantial drop".

The true investor scarcely ever has to sell his shares, and at all other times he is free di disregard the current price quotation. He need pay attention to it and act upon it only to the extent that it suits his book, and no more. Thus the investor who permits himself to be stampeded or unduly worried by unjustified market declines in his holdings is perversely transforming his basic advantage into a basic disadvantage. That man would be better off if his stocks had no market quotation at all, for he would then be spared the mental anguish caused him by other persons' mistakes of judgement.

Why could the typical investor expect any better success in trying to buy at low levels and sell at high levels than in trying to forecast what the market is going to do? Because if he does the former he acts only after the market has moved down into buying levels or up into selling levels. His role is not that of a prophet but of a businessman seizing clearly evident investment opportunities. He is not trying to be smarter than his fellow investors but simply trying to be less irrational than the mass of speculators who insist on buying after the market advances and selling after it goes down. If the market persists in behaving foolishly, all he seems to need is ordinary common sense in order to exploit its foolishness.