This train of thought starts with a problem in financial portfolio performance analysis. The problem is in measuring performance volatility — the extent to which results may be expected to vary from year to year from the average performance (variance, in technical terms, or its square root, standard deviation).

I was at a conference yesterday and attended a presentation on evaluating portfolio managers. The presenter stated that mathematical analysts prefer to have as many years of data as possible when computing variance. Statisticians prefer to have 75 or 100 data points or more. Obviously an impossibility when measuring manager performance. But the longer the manager has been managing that portfolio, the more reliable the variance statistics will be.

And that’s when the thought struck me: How do we know, even if the manager has been managing this portfolio for a long time, that he’s the same person he was when he started? In fact, how do we ever know if a person is the same person they were some time ago?

In fact, maybe they’re not. There’s the (true) story of a portfolio manager who threw in the towel and headed off to New Mexico on a motorcycle. Maybe performance measurement is only a useful fiction.

Last night, while cleaning my desk and throwing out papers that were over a year old and no longer relevant as well as papers just as old that I enjoyed but whose enjoyment value no longer exceeded their clutter annoyance, I thought of Adam. How neat he always kept his room, periodically going through his books and music, giving away or selling those he no longer used. How much of his once-extensive Depeche Mode collection does he still own? How much Moody Blues or Leonard Cohen does he still listen to? How much of what he acquired in high school or middle school does he still have? Probably only a small fraction. He’s different now than he was then.

Maybe everyone is different. Maybe all that keeps up the illusion of sameness is some kind of personality inertia. Maybe it’s just that some of us are more nostalgic about the tangible and intangible mementos of earlier selves; and others are more clutter-intolerant.