The true investor scarcely ever is forced to sell his shares, and at all other times he is free to disregard the current price quotation. He need pay attention to it and act upon it only to the extent that it suits his book, no more. Thus the investor who permits himself to be stampeded or undly 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 mental anguish cause him by other persons’ mistake of judgment.
– Benjaman Graham in The Intelligent Investor