How do you measure something that has not happened yet?
You can't. Data is always historical. We can only measure what is.
The future, we can calculate, if it is repeating, or expect, if it is not.
Science and engineering calculate the future by studying the past. Scientists and engineers study data collected on past events to discover patterns that repeat at regular intervals under specified circumstances. When the circumstances change, they change their patterns. In some circumstances, their patterns become only expectations. They expect, but they do not really know. The patterns don't really repeat, at least not in ways that we can see. The world, or at least our knowledge of the world, is still evolving.
Such is the case with financial valuations within the current default form of investment as securities trading. The price at which securities trade is always only an expectation, a guess at what is not yet known, because it has not yet happened.
The theory posits that the market will always get the price right. Experience shows the market always gets the price more or less right. That "more or less" makes the difference between profit and loss in a trading position. Market professionals call it "risk".
Portfolio theory holds that price risk is random, and this randomness can be canceled out by the Law of Large numbers: repeat the same action enough times and random outcomes will settle down into a predictable pattern. Diversify across enough trading positions, and volatility will settle down into capital appreciation over time.
Experience shows that expectations are not wrong through random error. They are wrong because we cannot see that far into the future.
Things change, and we adapt. We learn new things. We shape new possibilities. We make new choices. We take new actions. We create our own future.
Our future is not repeating. It is evolving. Evolution cannot be calculated. It must be experienced.
The current default form of investment as securities trading is built on calculations of that which cannot be calculated. It's valuations are always only more or less right, which means they are always also more or less wrong. They are not really calculations based on repeating patterns. They are calculations based on expectations for what will happen, but has not happened yet.
For us, as individuals, our time is limited. At some point, our clock runs out, and the expectations stop. Relative to investment, we only care about the future for so long.
But what about an evergreen trust? The clock for a pension plan never runs out. It just keeps going. So, the expectations never stop.
This is not easy to see at first, but it merits attention. Think about it.