“The market,” said J.P. Morgan, when asked for his opinion on what the market would do, “will fluctuate.”

Truer words were never spoken, but the depth of the truism as well is interesting. One implication of this observation – that prices will vary – is that the patient investor should mostly ignore noise in the markets. Ben Graham went further; he proposed thinking about a hypothetical “Mister Market,” who every day would offer to buy your stocks or sell you some more. On some days, Mister Market is fearful and offers to sell you stocks at a terrific discount; on other days, he is ebullient and offers to buy your holdings at far more than they are worth. Graham argued that this can only be a positive for an investor who knows the value of the business he holds. He can sell it if Mister Market is paying too much, or buy it if Mister Market is selling it too cheaply.

Graham did not give enough weight to momentum, as opposed to value – the idea that Mister Market might be paying too much today, but if you sell your holdings to him today, then you might miss the opportunity to sell them to him next year for double the stupid price. And, over the last couple of decades, momentum has become far more important to most investors than has value. (I blame CNBC, but that’s a different story).

In either case, the point is important – if you know what you own, and why you own it, and even better if you have an organized framework for thinking about the investment that is time-independent (that is, it doesn’t depend on how you feel today or tomorrow), then the zigs and zags don’t matter much to you in terms of your existing investments.

(As for future investments, young people should prefer declining asset markets, since they will be investing for long periods and should prefer lower prices to buy rather than higher prices; on the other hand, retirees should prefer rising asset prices, since they will be net sellers and should prefer higher prices to lower prices. In practice, everyone seems to like higher prices even though this is not rational in terms of one’s investing life.)