Uncertainty-What It Means and What It Doesn’t

It is in the admission of ignorance and the admission of uncertainty that there is a hope for the continuous motion of human beings in some direction that doesn’t get confined, permanently blocked, as it has so many times before in various periods in the history of man.
Richard P. Feynman

I’ve written before about uncertainty and how it affects medical decision making and also about why pet owners often prefer simple, unambiguous answers to the messy uncertainties of reality. But I’d like to take a slightly different perspective on the issue of uncertainty in science and medicine. The fact that science is incomplete and imperfect is undeniable. Unfortunately, some people take this fact as justification for throwing out the whole enterprise of trying to understand and acquire real facts and knowledge. If vaccines don’t protect us completely, this means they are useless. If science doesn’t know everything, than anything must be possible. These naïve arguments ignore the very clear fact that uncertainty and the impossibility of knowledge are not the same thing.

There are different kind of uncertainty in science. One is the uncertainty of what we don’t know. A big part of what makes science fun is that the universe is so vastly more complex than our little brains that we are never going to be in danger of running out of new things to learn and discover. I like to imagine my own ignorance as a vast abyss yawing before me. Every day I throw in a few grains of sand, but it shows no sign of being filled in. So there is no denying that the surprising and unexpected is out there waiting for us.

But that is not the same thing as saying anything we can imagine is likely to be true. Most of our guesses about the nature of reality turn out to be wrong. Until science came along, this left us fighting over belief systems and led to a bewildering proliferation of different, interesting, and usually mutually incompatible mythologies to explain the world. Now, science is creating the kind of knowledge that works everywhere, in every culture, and that endures through time. Sure, such secure knowledge is only tidbits compared to the vastness of reality, and there is plenty that falls by the wayside. But never before have we been able to have even this much enduring knowledge. Barring the complete collapse of human civilization, we are always going to know that the heart circulates blood, that emotions live in the brain not the heart, that smallpox used to be caused by a bacterium, and so on. What we don’t know is an opportunity, not an invalidation of what we do know.

Another kind of uncertainty, though, is the uncertainty about what we know. Scientists like to say that all knowledge is provisional, tentative and subject to revision. This is true, but non-scientists tend to overread this and believe it means all knowledge is ultimately just opinion and is unreliable. Sure, theories of gravity have changed from Newton’s day to our, but if you have to leave your 8th floor apartment, you can rely confidently on the knowledge that it is better to do so by the stairway than the window. It is possible that gravity will stop working tomorrow, but I wouldn’t bet on it. So while proponents of bizarre ideas like to make much of the fact that scientific explanations are “only theories,” they ignore the fact that those ideas that endure and are refined over time can reach a point where having confidence in their truth makes a lot more sense than doubting them.

A different flavor of uncertainty about the known is statistical or probabilistic uncertainty. I recently put my age, gender, total cholesterol level, and a few other factors into a nifty little calculator that told me I have a 3% chance of dying of a heart attack in the next 10 years. So does anyone know if I’m going to die of a heart attack or not? Can anyone tell me with certainty that I will if I stop taking my fish oil and baby aspirin every day, but I’m safe if I keep taking them? No, of course not. Much scientific research generates knowledge and conclusions that are statistical, that apply reliably to groups but don’t give precise predictions for individuals. So, does this mean such knowledge is worthless for helping us decide what to do as individuals? Of course not.

If you go to a casino in Vegas and play roulette, the odds are you’re going to lose. Sure, you could win. Some people do. But most people don’t. This is a truth, though it is only a statistical or probabilistic truth. So is it worthless in helping you decide whether or not to bet the farm on red? Not at all. Casinos make lots of money betting against you, and you’re a lot more likely to be able to afford that Winnebago when you retire if you play the odds and don’t play roulette. Even though statistical truths apply imperfectly to the individual, they are real and useful guides for our choices. The uncertainty of probabilities does not justify ignoring the odds and doing whatever we like.

Uncertainty is inevitable, in science as in all areas of human life. But this doesn’t mean knowledge is an illusion and blind belief is as good as facts in deciding what to do or not to do. Human beings have changed our planet and our own lives, both for the better and for the worse, through the power of discovering and applying knowledge about the physical world. Such knowledge is limited, incomplete, tenuous, and a damn site better than guessing or hoping. And science has generated more reliable, trustworthy knowledge in the past couple centuries than hunches, guesses, and trial-and-error managed in all the rest of human history. Place your bets, ladies and gentlemen. I’m placing mine on science.

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