1. I agree that experts have greater credibility than nonexperts when both are observers of something; but how can I trust an expert who wasn't there over a nonexpert who was, or even over my own eyes?
2. When can all the experts in a field be wrong about something, and what do we do about that possibility?

1. I agree that experts have greater credibility than nonexperts when both are observers of something; but how can I trust an expert who wasn't there over a nonexpert who was, or even over my own eyes?

In many cases there is nothing like being there. Suppose a locksmith installs a new lock in your door, but two days later your key won't open it. You call to complain, and the locksmith says, "Impossible. Those locks always work." That expert assurance is not worth much when you've been battling the lock for fifteen minutes.

That example doesn't quite prove enough though, mainly because a stuck lock is the kind of thing you don't need experts to detect. A surprising number of what we think of as direct observations actually involve theoretical assumptions or generalizations about experience. For instance, if you stand watching the eastern horizon at dawn, you will see the sun come up. What could be more obvious? So, many people who first heard of the Copernican theory that the earth goes around the sun would have thought, "What does Copernicus know? I saw the sun go up with my own eyes. Was he there?" We would have to answer them that the "observation" of sunrise amounted to an interpretation, based on the assumption that the earth stood still.

Closer to home, a lot of men believe that their beards grow faster if they shave than if they don't. Five or six days of not shaving simply doesn't appear to produce the quantity of facial hair that comes off in five or six days' shaves. (Almost no man ever runs the experiment of shaving one side of his face every day, washing and collecting the shavings, and letting the other side grow for a year, then washing and collecting those shavings.) A doctor or biologist could explain that hair follicles grow at a steady pace regardless of how often they are clipped. Despite the conflict with observation, the scientific explanation simply has more credibility.

2. When can all the experts in a field be wrong about something, and what do we do about that possibility?

Drastic changes have a way of leaving experts high and dry, especially when they hit scientific theories. Whether it is the abandonment of Newton by Einstein and other physicists, or the change in geology to say that continents move, a ground-level scientific revolution renders the majority of experts in its field suddenly and retroactively wrong.

But when it takes something as dramatic as a scientific revolution to make experts collectively wrong, suspended judgment is the wrong response to an expert's claim. It is one thing not to buy one psychologist's account of forgetfulness when other psychologists reject it; it is quite another thing to shrug off a chemist's analysis of some material on the grounds that the next century could bring a revision of chemical theory. When a field of experts speaks with near-unanimity, you need a substantial reason not to believe them. The mere possibility of some new theory or other does not count as a substantial reason, unless you have worked out a coherent new chemical or physical theory by yourself (and good luck with that).

We may say more broadly that suspending judgment is not a fail-safe response to claims. When different experts make competing assertions about which foods belong in a healthy diet, you have no choice but to agree with some of them—you have to eat something. Nor are you always justified in suspending judgment, as the example from science shows.

This discussion raises two related issues. First, different fields have different relationships to facts of the matter, and the claims of their experts call for different responses. English professors may all agree about which novels in English are the great books, but that agreement isn't worth much just as a fact (knowing which books are deemed great has value only if it gets us to read those books) and remains open to reevaluation without the kind of drastic change that happens in a scientific revolution.

Second, watch for mere references to experts. (See Chapter 4 on proof surrogates.) Although news sources almost never pass along false claims, they can use unnamed experts to give the claims one context or another. "Experts say that the radiation leak poses no health risks"; "Experts warn that reported embezzlements represent only 10 percent of corporate crimes." You may assume that the media evaluate the credentials of their sources. But when two experts might disagree on an issue, you need more than a word to assure you of the truth of a claim. Take an appeal to experts more seriously when the news source supplies you with a name.