15.2.4: Good Evidence

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Bradley H. Dowden
California State University Sacramento

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Many persons view science as some vast storehouse of knowledge. That is an accurate view, but we also should view science as a way of getting to that knowledge. This latter way of looking at science is our primary concern in this chapter. In acquiring knowledge, a good scientist adopts a skeptical attitude that says, "I won't believe you unless you show me some good evidence." Why do scientists have this attitude? Because it is so successful. Scientists who are so trusting that they adopt beliefs without demanding good evidence quickly get led astray; they soon find themselves believing what is false, which is exactly what science is trying to avoid.

What constitutes good evidence? How do you distinguish good from bad evidence? It’s not like the evidence appears with little attached labels of “good” and “bad.” Well, if a scientist reports that tigers won't eat vegetables, the report is about a phenomenon that is repeatable—namely, tiger meals. If the evidence is any good, and the phenomenon is repeatable, the evidence should be, too. That is, if other scientists rerun the first scientist's tests, they should obtain the same results. If not, the evidence was not any good. The moral here is that reproducible evidence is better than evidence that can't be reproduced. The truth is able to stand up to repeated tests, but falsehood can eventually be exposed. That is one of the major metaphysical assumptions of contemporary science.

A scientist who appreciates good evidence knows that having anecdotal evidence isn't as good as having a wide variety of evidence. For example, suppose a scientist reads an article in an engineering journal saying that tests of 300 randomly selected plastic ball bearings showed the bearings to be capable of doing the job of steel ball bearings in the electric windows of Honda cars.

The journal article reports on a wide variety of evidence, 300 different ball bearings. If a scientist were to hear from one auto mechanic that plastic bearings didn't hold up on the car he repaired last week, the scientist won't be a good logical reasoner if he (or she) immediately discounted the wide variety of evidence and adopted the belief of the one auto mechanic. We logical reasoners should trust the journal article over the single anecdote from the mechanic, although the mechanic's report might alert us to be on the lookout for more evidence that would undermine the findings of the journal article. One lemon does not mean that Honda’s electric windows need redesigning. If you discount evidence arrived at by systematic search, or by testing, in favor of a few firsthand stories, you’ve committed the fallacy of overemphasizing anecdotal evidence.