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Verification and falsification

The distinction between verification and falsification: the fallacy of affirming the consequent in practice.
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OK, in the last clip we set out the scientific method. Identify a problem or pose a question, devise hypotheses to explain an event, or a phenomenon, or observations, derive a test for the hypothesis, perform the test, accept or reject the hypothesis. But as Patrick pointed out, there was a logical problem with the last step. Logic tells us that tests can lead us to reject the hypothesis, but they can’t settle that it’s true. That’s the distinction between verification and falsification. Let’s look at the logic of that. Suppose we know that if a hypothesis H is correct, then some consequence C will occur in our test.
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If we run the test and the consequence C doesn’t occur, we have reason to reject the hypothesis H. If H, then C, not C, therefore not H. If the supernova hypothesis H was true, then we’d find plutonium-244 in the strata around the end of the cretaceous. That’s our C. But there was no plutonium-244 in the strata around the end of the Cretaceous. Not C, therefore the supernova hypothesis is false, not H. But suppose we know that if hypothesis H is correct then consequence C will occur in our test. If we run the test and C does occur, we can’t be certain that H is correct. If H, then C, C therefore H, that’s invalid. It’s affirming the consequent.
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Patrick, did you tell them about affirming the consequent? No, but it’s in the article on logical fallacies. Still, let’s remind ourselves why affirming the consequent is an invalid argument form. The problem is that if H then C is not equivalent to if and only if H then C. And because if is not equivalent to if and only if, if H then C is consistent with C but not H, C might be caused by something other than the explanation proposed by H. So if we go back to our poor old dinosaurs for instance, we will see that there are other plausible explanations for the observations. Thanks, Tim. And now to San Francisco.
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According to a new study, volcanic activity in what is now India, and not an asteroid, may have killed the dinosaurs. Tens of thousands of years of lava flowing from the Deccan Traps, a volcanic region near Mumbai and present day India, may have spewed poisonous levels of sulphur and carbon dioxide into the atmosphere causing the mass extinction through the resulting global warming and ocean acidification. These are the findings being presented on Wednesday at the annual meeting of the American Geophysical Union in the latest volley in an ongoing debate over whether an asteroid or volcanism killed off the dinosaurs about 65 million years ago in what is known as the KT Extinction. Back to you, Tim.
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So why are we even entertaining the volcano hypothesis? Didn’t the asteroid hypothesis win? Well, the asteroid hypothesis won only in the sense that the test showed that if it were true, then it would explain the observations. It didn’t show that those observations would be explained only by the asteroid hypothesis. If it had failed our tests, we’d know that it was false. But just because it passed them, we can’t say definitively that it’s true. That’s the distinction between verification, showing something to be true, and falsification, showing something to be false. That’s why we have to avoid the fallacy of affirming the consequent. And that’s why we remain open to other possibilities.
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Remember that if the volcano theory is a good scientific hypothesis, it’ll be testable. Is it? Well, probably. It’s easy enough to think of tests. Here’s one. What would the geological record show if there had been a sustained period of volcanic activity toward the end of the Cretaceous period? And so science proceeds ruling out some hypotheses, gathering new evidence, doing new tests, trying to falsify new and alternative hypotheses.

Our tests showed that if the asteroid hypothesis were true, it would explain our observations: the fossilised dinosaur bones in rock strata up until the end of the Cretaceous period, but not after; the absence of many other types of terrestrial and marine fossils after the end of the Cretaceous period; high iridium levels in rock strata at the boundary between the Cretaceous and the Tertiary periods. If the hypothesis had failed those tests, we’d know that it was false. We cannot say definitively that it’s true, however, just because it passed them. The tests show that if the hypothesis were true it would explain the observations, but they do not show that those observations would be explained only by the asteroid hypothesis. That’s the distinction between verification, showing something to be true, and falsification, showing something to be false. That’s why we have to avoid the fallacy of affirming the consequent. And that’s why we remain open to other possibilities.

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Logical and Critical Thinking

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